compositor worker: Use a WebThread for the compositor thread.

content/renderer/render_thread_impl.cc

Index: content/renderer/render_thread_impl.cc
diff --git a/content/renderer/render_thread_impl.cc b/content/renderer/render_thread_impl.cc
index db1591c407bfda3a28758117657834259d1a42cc..c9b4135f82263858842399ed203bd2966304189a 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -895,7 +895,7 @@ void RenderThreadImpl::Shutdown() {
 
   media_thread_.reset();
 
-  blink_platform_impl_->set_compositor_thread(nullptr);
+  blink_platform_impl_->reset_compositor_thread();
 
   compositor_thread_.reset();
 
@@ -1148,14 +1148,8 @@ void RenderThreadImpl::InitializeCompositorThread() {
   }
 #endif
   if (!compositor_task_runner_.get()) {
-
-    compositor_thread_.reset(new base::Thread("Compositor"));
-    base::Thread::Options compositor_thread_options;
-#if defined(OS_ANDROID)
-    compositor_thread_options.priority = base::ThreadPriority::DISPLAY;
-#endif
-    compositor_thread_->StartWithOptions(compositor_thread_options);
-    compositor_task_runner_ = compositor_thread_->task_runner();
+    compositor_thread_ = blink_platform_impl_->createCompositorThread();
+    compositor_task_runner_ = compositor_thread_->TaskRunner();
     compositor_task_runner_->PostTask(
         FROM_HERE,
         base::Bind(base::IgnoreResult(&ThreadRestrictions::SetIOAllowed),