Disable forwarding tasks to the Blink scheduler

Disable forwarding tasks to the Blink scheduler to avoid some
regressions which it has introduced.

BUG=391005,415758,415478,412714,416362,416827,417608
TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/609483002

Cr-Commit-Position: refs/heads/master@{#296916}

content/renderer/render_thread_impl.cc

@@ -852,9 +852,7 @@ void RenderThreadImpl::EnsureWebKitInitialized() {
 
   webkit_platform_support_.reset(new RendererWebKitPlatformSupportImpl);
   blink::initialize(webkit_platform_support_.get());
-  main_thread_compositor_task_runner_ =
-      make_scoped_refptr(new SchedulerProxyTaskRunner<
-          &blink::WebSchedulerProxy::postCompositorTask>());
+  main_thread_compositor_task_runner_ = base::MessageLoopProxy::current();
 
   v8::Isolate* isolate = blink::mainThreadIsolate();