Reland "Fix flakly content_browsertests"

This is a reland of d2d3af26

Also we now call
base::PostTaskAndroid::SignalNativeSchedulerShutdown() which on the
Java side has the following comment
"This is here to make C++ tests work"

Probably a good idea to call it :)

Also we no longer call ResetForTesting() but Shutdown instead. As this
what happens in prod.

Original change's description:
> Fix flakly content_browsertests
>
> Looks like BrowserTaskExecutor is being deleted while the IO thread is
> still running and thus capable of posting tasks. Also  browser tests are
> like prod, we shouldn't manually shut down pools and schedulers.
>
> Bug: 963702
> Change-Id: I1cb953acc4b495631fe269af9e06b94fbe5f2a52
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628748
> Commit-Queue: Carlos Caballero <carlscab@google.com>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Reviewed-by: Alex Clarke <alexclarke@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#667097}

Bug: 963702
Change-Id: I88f2b8d418eeb1a9a44f8f17bd5c1147993e8e71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1651725Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#682624}

content/browser/scheduler/browser_task_executor.h

@@ -62,6 +62,8 @@ class CONTENT_EXPORT BrowserTaskExecutor : public base::TaskExecutor {
   //
   // Attention: This method can only be called once (as there must be only one
   // IO thread).
+  // Attention: Must be called after Create()
+  // Attention: Can not be called after Shutdown() or ResetForTesting()
   static std::unique_ptr<BrowserProcessSubThread> CreateIOThread();
 
   // Enables non best effort queues on the IO thread. Usually called from
@@ -83,7 +85,9 @@ class CONTENT_EXPORT BrowserTaskExecutor : public base::TaskExecutor {
 
   // Winds down the BrowserTaskExecutor, after this no tasks can be executed
   // and the base::TaskExecutor APIs are non-functional but won't crash if
-  // called.
+  // called. In unittests however we need to clean up, so
+  // BrowserTaskExecutor::ResetForTesting should be
+  // called (~TestBrowserThreadBundle() takes care of this).
   static void Shutdown();
 
   // Unregister and delete the TaskExecutor after a test.

content/public/test/browser_test_base.cc

@@ -69,6 +69,7 @@
 #include "ui/gl/gl_switches.h"
 
 #if defined(OS_ANDROID)
+#include "base/android/task_scheduler/post_task_android.h"
 #include "components/discardable_memory/service/discardable_shared_memory_manager.h"  // nogncheck
 #include "content/app/mojo/mojo_init.h"
 #include "content/app/service_manager_environment.h"
@@ -461,20 +462,22 @@ void BrowserTestBase::SetUp() {
     discardable_shared_memory_manager.reset();
     spawned_test_server_.reset();
   }
-  BrowserTaskExecutor::ResetForTesting();
+
+  base::PostTaskAndroid::SignalNativeSchedulerShutdown();
+  BrowserTaskExecutor::Shutdown();
 
   // Normally the BrowserMainLoop does this during shutdown but on Android we
   // don't go through shutdown, so this doesn't happen there. We do need it
   // for the test harness to be able to delete temp dirs.
   base::ThreadRestrictions::SetIOAllowed(true);
-#else
+#else   // defined(OS_ANDROID)
   auto ui_task = std::make_unique<base::Closure>(base::Bind(
       &BrowserTestBase::ProxyRunTestOnMainThreadLoop, base::Unretained(this)));
   GetContentMainParams()->ui_task = ui_task.release();
   GetContentMainParams()->created_main_parts_closure =
       created_main_parts_closure.release();
   EXPECT_EQ(expected_exit_code_, ContentMain(*GetContentMainParams()));
-#endif
+#endif  // defined(OS_ANDROID)
   TearDownInProcessBrowserTestFixture();
 }