mus: Avoid a crash when the gpu mojom connection is lost.

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 7b9801a18dbfadde5bad8f9646fb56e3b5f1b551..f85e95829608371ce830d9fdeda5eb8f90d593be 100644
--- a/content/renderer/render_thread_impl.cc
+++ b/content/renderer/render_thread_impl.cc
@@ -1863,10 +1863,14 @@ RenderThreadImpl::CreateCompositorFrameSink(
 #if defined(USE_AURA)
   if (!use_software && IsRunningInMash() &&
       !command_line.HasSwitch(switches::kNoUseMusInRenderer)) {
+    scoped_refptr<gpu::GpuChannelHost> channel = EstablishGpuChannelSync();
+    // If the channel could not be established correctly, then return null. This
+    // would cause the compositor to wait and try again at a later time.
+    if (!channel)
+      return nullptr;
     return RendererWindowTreeClient::Get(routing_id)
         ->CreateCompositorFrameSink(
-            frame_sink_id,
-            gpu_->CreateContextProvider(EstablishGpuChannelSync()),
+            frame_sink_id, gpu_->CreateContextProvider(std::move(channel)),
             GetGpuMemoryBufferManager());
   }
 #endif