Revert "[blink scheduler] Add blink::Thread::ThreadHandle."

This reverts commit e66dcc06.

Reason for revert: Thread handle != thread id on Linux
crbug.com/1158103

Original change's description:
> [blink scheduler] Add blink::Thread::ThreadHandle.
>
> The thread handle should be used instead of the thread id for thread
> manipulations. It is available immediately after thread creation,
> unlike the thread id which is available only after the main function
> is entered. Also, on Windows, the thread handle remains valid after the
> thread has exited.
>
> This is a prerequisite to remove the wait after compositor thread
> initialization
> https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/platform/scheduler/common/thread.cc;l=96;drc=5b1ca7b1087e13d963bc4058d9406a838fb7f9ca
>
> Bug: 1080709
> Change-Id: Ia0b72aa1b3a86454cffa774131944db87ced0843
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2578172
> Reviewed-by: Alexander Timin <altimin@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Commit-Queue: Kentaro Hara <haraken@chromium.org>
> Auto-Submit: François Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#835544}

TBR=gab@chromium.org,fdoray@chromium.org,haraken@chromium.org,altimin@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1080709, 1158103
Change-Id: Ib4bd9113ad8a72fe4e9215989e07986d3b357c20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593833Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837302}

base/threading/simple_thread.cc

@@ -60,11 +60,6 @@ PlatformThreadId SimpleThread::tid() {
   return tid_;
 }
 
-PlatformThreadHandle SimpleThread::handle() const {
-  DCHECK(!thread_.is_null());
-  return thread_;
-}
-
 bool SimpleThread::HasBeenStarted() {
   return event_.IsSignaled();
 }

base/threading/simple_thread.h

@@ -112,10 +112,6 @@ class BASE_EXPORT SimpleThread : public PlatformThread::Delegate {
   // be called before HasBeenStarted() returns True.
   PlatformThreadId tid();
 
-  // Returns the thread handle. Valid only if the thread is joinable, started
-  // and not yet joined.
-  PlatformThreadHandle handle() const;
-
   // Returns True if the thread has been started and initialized (i.e. if
   // ThreadMain() has run). If the thread was started with StartAsync(), but it
   // hasn't been initialized yet (i.e. ThreadMain() has not run), then this will

content/renderer/renderer_blink_platform_impl.cc

@@ -234,12 +234,11 @@ RendererBlinkPlatformImpl::WrapSharedURLLoaderFactory(
 }
 
 void RendererBlinkPlatformImpl::SetDisplayThreadPriority(
-    base::PlatformThreadHandle thread_handle) {
+    base::PlatformThreadId thread_id) {
 #if defined(OS_LINUX) || defined(OS_CHROMEOS)
   if (RenderThreadImpl* render_thread = RenderThreadImpl::current()) {
-    // On Linux and ChromeOS, the thread handle is equivalent to the thread id.
     render_thread->render_message_filter()->SetThreadPriority(
-        thread_handle.platform_handle(), base::ThreadPriority::DISPLAY);
+        thread_id, base::ThreadPriority::DISPLAY);
   }
 #endif
 }

content/renderer/renderer_blink_platform_impl.h

@@ -183,8 +183,7 @@ class CONTENT_EXPORT RendererBlinkPlatformImpl : public BlinkPlatformImpl {
       const blink::WebURL& top_document_web_url) override;
   gpu::GpuMemoryBufferManager* GetGpuMemoryBufferManager() override;
   blink::WebString ConvertIDNToUnicode(const blink::WebString& host) override;
-  void SetDisplayThreadPriority(
-      base::PlatformThreadHandle thread_handle) override;
+  void SetDisplayThreadPriority(base::PlatformThreadId thread_id) override;
   blink::BlameContext* GetTopLevelBlameContext() override;
   std::unique_ptr<blink::WebDedicatedWorkerHostFactoryClient>
   CreateDedicatedWorkerHostFactoryClient(

third_party/blink/public/platform/platform.h

@@ -392,7 +392,7 @@ class BLINK_PLATFORM_EXPORT Platform {
   // increase the nice value, so we need to ask the browser process). This
   // function is only called from the main thread (where InitializeCompositor-
   // Thread() is called).
-  virtual void SetDisplayThreadPriority(base::PlatformThreadHandle) {}
+  virtual void SetDisplayThreadPriority(base::PlatformThreadId) {}
 
   // Returns a blame context for attributing top-level work which does not
   // belong to a particular frame scope.

third_party/blink/renderer/platform/scheduler/common/thread.cc

@@ -102,7 +102,7 @@ void Thread::CreateAndSetCompositorThread() {
     // This is not possible inside the sandbox, so ask the browser to do it.
     // TODO(spang): Check if we can remove this on non-Chrome OS builds.
     Platform::Current()->SetDisplayThreadPriority(
-        GetCompositorThread()->ThreadHandle());
+        GetCompositorThread()->ThreadId());
   }
 }
 

third_party/blink/renderer/platform/scheduler/main_thread/main_thread.cc

@@ -14,8 +14,7 @@ namespace scheduler {
 MainThread::MainThread(MainThreadSchedulerImpl* scheduler)
     : task_runner_(scheduler->DefaultTaskRunner()),
       scheduler_(scheduler),
-      thread_id_(base::PlatformThread::CurrentId()),
-      thread_handle_(base::PlatformThread::CurrentHandle()) {}
+      thread_id_(base::PlatformThread::CurrentId()) {}
 
 MainThread::~MainThread() = default;
 
@@ -23,10 +22,6 @@ blink::PlatformThreadId MainThread::ThreadId() const {
   return thread_id_;
 }
 
-base::PlatformThreadHandle MainThread::ThreadHandle() const {
-  return thread_handle_;
-}
-
 blink::ThreadScheduler* MainThread::Scheduler() {
   return scheduler_;
 }

third_party/blink/renderer/platform/scheduler/main_thread/main_thread.h

@@ -7,7 +7,6 @@
 
 #include "base/memory/scoped_refptr.h"
 #include "base/single_thread_task_runner.h"
-#include "base/threading/platform_thread.h"
 #include "third_party/blink/renderer/platform/platform_export.h"
 #include "third_party/blink/renderer/platform/scheduler/public/thread.h"
 
@@ -27,7 +26,6 @@ class PLATFORM_EXPORT MainThread : public Thread {
   // Thread implementation.
   ThreadScheduler* Scheduler() override;
   PlatformThreadId ThreadId() const override;
-  base::PlatformThreadHandle ThreadHandle() const override;
   scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() const override;
 
   void AddTaskTimeObserver(base::sequence_manager::TaskTimeObserver*) override;
@@ -35,10 +33,9 @@ class PLATFORM_EXPORT MainThread : public Thread {
       base::sequence_manager::TaskTimeObserver*) override;
 
  private:
-  const scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
-  MainThreadSchedulerImpl* const scheduler_;  // Not owned.
-  const PlatformThreadId thread_id_;
-  const base::PlatformThreadHandle thread_handle_;
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
+  MainThreadSchedulerImpl* scheduler_;  // Not owned.
+  PlatformThreadId thread_id_;
 };
 
 }  // namespace scheduler

third_party/blink/renderer/platform/scheduler/public/thread.h

@@ -26,12 +26,10 @@
 #define THIRD_PARTY_BLINK_RENDERER_PLATFORM_SCHEDULER_PUBLIC_THREAD_H_
 
 #include <stdint.h>
-
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/scoped_refptr.h"
 #include "base/task/task_observer.h"
-#include "base/threading/platform_thread.h"
 #include "base/threading/thread.h"
 #include "third_party/blink/renderer/platform/platform_export.h"
 #include "third_party/blink/renderer/platform/scheduler/public/thread_type.h"
@@ -128,10 +126,7 @@ class PLATFORM_EXPORT Thread {
   }
 
   bool IsCurrentThread() const;
-  virtual PlatformThreadId ThreadId() const { return base::kInvalidThreadId; }
-  virtual base::PlatformThreadHandle ThreadHandle() const {
-    return base::PlatformThreadHandle();
-  }
+  virtual PlatformThreadId ThreadId() const { return 0; }
 
   // TaskObserver is an object that receives task notifications from the
   // MessageLoop.

third_party/blink/renderer/platform/scheduler/worker/worker_thread.cc

@@ -76,10 +76,6 @@ blink::PlatformThreadId WorkerThread::ThreadId() const {
   return thread_->tid();
 }
 
-base::PlatformThreadHandle WorkerThread::ThreadHandle() const {
-  return thread_->handle();
-}
-
 blink::ThreadScheduler* WorkerThread::Scheduler() {
   return thread_->GetNonMainThreadScheduler();
 }

third_party/blink/renderer/platform/scheduler/worker/worker_thread.h

@@ -42,7 +42,6 @@ class PLATFORM_EXPORT WorkerThread : public Thread {
   void Init() override;
   ThreadScheduler* Scheduler() override;
   PlatformThreadId ThreadId() const override;
-  base::PlatformThreadHandle ThreadHandle() const override;
   scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() const override;
 
   scheduler::NonMainThreadSchedulerImpl* GetNonMainThreadScheduler() {