Fix GpuChannelHost destruction and races

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 207724bdc327a6dbc427a7ab49d7ca2987196434..5d010b75827b1bf3735295d999c46d5b7f4014de 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -790,6 +790,7 @@ void RenderThreadImpl::Shutdown() {
 
   // Context providers must be released prior to destroying the GPU channel.
   gpu_va_context_provider_ = nullptr;
+  shared_main_thread_contexts_ = nullptr;
 
   if (gpu_channel_.get())
     gpu_channel_->DestroyChannel();
@@ -1605,6 +1606,7 @@ GpuChannelHost* RenderThreadImpl::EstablishGpuChannelSync(
       return gpu_channel_.get();
 
     // Recreate the channel if it has been lost.
+    gpu_channel_->DestroyChannel();
     gpu_channel_ = NULL;
   }