Forward compositor and input tasks to the Blink scheduler

The re-entrancy issues with the Blink scheduler have been resolved, so
we can forward compositor and input tasks to it again -- if the
scheduler is enabled.

Depends on https://codereview.chromium.org/621363002.

BUG=391005
TEST=No instances of "Scheduler::executeHighPriorityTasks" found
in trace when running chrome with --disable-blink-scheduler.

Review URL: https://codereview.chromium.org/629243003

Cr-Commit-Position: refs/heads/master@{#299105}

content/renderer/render_thread_impl.cc

@@ -851,7 +851,6 @@ void RenderThreadImpl::EnsureWebKitInitialized() {
 
   blink_platform_impl_.reset(new RendererBlinkPlatformImpl);
   blink::initialize(blink_platform_impl_.get());
-  main_thread_compositor_task_runner_ = base::MessageLoopProxy::current();
 
   v8::Isolate* isolate = blink::mainThreadIsolate();
 
@@ -861,6 +860,14 @@ void RenderThreadImpl::EnsureWebKitInitialized() {
 
   const CommandLine& command_line = *CommandLine::ForCurrentProcess();
 
+  if (command_line.HasSwitch(switches::kDisableBlinkScheduler)) {
+    main_thread_compositor_task_runner_ = base::MessageLoopProxy::current();
+  } else {
+    main_thread_compositor_task_runner_ =
+        make_scoped_refptr(new SchedulerProxyTaskRunner<
+            &blink::WebSchedulerProxy::postCompositorTask>());
+  }
+
   bool enable = !command_line.HasSwitch(switches::kDisableThreadedCompositing);
   if (enable) {
 #if defined(OS_ANDROID)