Revert of Pending tasks in a message loop should be deleted before shutting...

Revert of Pending tasks in a message loop should be deleted before shutting down Blink (patchset #7 id:120001 of https://codereview.chromium.org/583043005/)

Reason for revert:
Speculative revert. Likely cause of crbug.com/418206

Original issue's description:
> Pending tasks in a message loop should be deleted before shutting down Blink
>
> Currently Blink is shut down before all the pending tasks in the message loop are deleted. This is problematic in Oilpan because a destructor of the pending tasks can touch Oilpan objects. Because Oilpan is already detached from the renderer thread at that point, touching Oilpan objects in the destructor leads to a crash. (See the bug report for a concrete scenario.)
>
> To prevent Blink objects from getting accessed after Blink is shut down, this CL deletes all pending tasks in a message loop before shutting down Blink.
>
> BUG=411026
> TEST=None. I cannot reproduce the crash.
>
> Committed: https://crrev.com/fdd5612c20f777e1279efd7c1e99d82ed04afaaf
> Cr-Commit-Position: refs/heads/master@{#296697}

TBR=tkent@chromium.org,jochen@chromium.org,jamesr@chromium.org,ager@chromium.org,jar@chromium.org,haraken@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=411026

Review URL: https://codereview.chromium.org/608043002

Cr-Commit-Position: refs/heads/master@{#297022}

content/renderer/render_thread_impl.cc

@@ -403,13 +403,6 @@ RenderThreadImpl::RenderThreadImpl(const std::string& channel_name)
   Init();
 }
 
-RenderThreadImpl::RenderThreadImpl(
-    scoped_ptr<base::MessageLoop> main_message_loop)
-    : ChildThread(Options(ShouldUseMojoChannel())),
-      main_message_loop_(main_message_loop.Pass()) {
-  Init();
-}
-
 void RenderThreadImpl::Init() {
   TRACE_EVENT_BEGIN_ETW("RenderThreadImpl::Init", 0, "");
 
@@ -666,10 +659,6 @@ void RenderThreadImpl::Shutdown() {
 
   main_thread_compositor_task_runner_ = NULL;
 
-  // Shut down the message loop before shutting down Blink.
-  // This prevents a scenario where a pending task in the message loop accesses
-  // Blink objects after Blink shuts down.
-  main_message_loop_.reset();
   if (webkit_platform_support_)
     blink::shutdown();
 

content/renderer/render_thread_impl.h

@@ -120,8 +120,6 @@ class CONTENT_EXPORT RenderThreadImpl : public RenderThread,
   RenderThreadImpl();
   // Constructor that's used when running in single process mode.
   explicit RenderThreadImpl(const std::string& channel_name);
-  // Constructor that's used in RendererMain.
-  explicit RenderThreadImpl(scoped_ptr<base::MessageLoop> main_message_loop);
   virtual ~RenderThreadImpl();
   virtual void Shutdown() OVERRIDE;
 
@@ -527,11 +525,6 @@ class CONTENT_EXPORT RenderThreadImpl : public RenderThread,
   // GpuChannelHostFactory methods.
   scoped_refptr<base::MessageLoopProxy> io_message_loop_proxy_;
 
-  // The message loop of the renderer main thread.
-  // This message loop should be destructed before the RenderThreadImpl
-  // shuts down Blink.
-  scoped_ptr<base::MessageLoop> main_message_loop_;
-
   // A lazily initiated thread on which file operations are run.
   scoped_ptr<base::Thread> file_thread_;
 

content/renderer/renderer_main.cc

@@ -149,13 +149,12 @@ int RendererMain(const MainFunctionParams& parameters) {
   // needs to be backed by a Foundation-level loop to process NSTimers. See
   // http://crbug.com/306348#c24 for details.
   scoped_ptr<base::MessagePump> pump(new base::MessagePumpNSRunLoop());
-  scoped_ptr<base::MessageLoop> main_message_loop(
-      new base::MessageLoop(pump.Pass()));
+  base::MessageLoop main_message_loop(pump.Pass());
 #else
   // The main message loop of the renderer services doesn't have IO or UI tasks.
-  scoped_ptr<base::MessageLoop> main_message_loop(new base::MessageLoop());
+  base::MessageLoop main_message_loop;
 #endif
-  main_message_loop->AddTaskObserver(&task_observer);
+  main_message_loop.AddTaskObserver(&task_observer);
 
   base::PlatformThread::SetName("CrRendererMain");
 
@@ -199,7 +198,7 @@ int RendererMain(const MainFunctionParams& parameters) {
     // TODO(markus): Check if it is OK to unconditionally move this
     // instruction down.
     RenderProcessImpl render_process;
-    new RenderThreadImpl(main_message_loop.Pass());
+    new RenderThreadImpl();
 #endif
     bool run_loop = true;
     if (!no_sandbox) {
@@ -215,7 +214,7 @@ int RendererMain(const MainFunctionParams& parameters) {
     }
 #if defined(OS_POSIX) && !defined(OS_MACOSX)
     RenderProcessImpl render_process;
-    new RenderThreadImpl(main_message_loop.Pass());
+    new RenderThreadImpl();
 #endif
 
     base::HighResolutionTimerManager hi_res_timer_manager;