gpu: Add flag for enabling asynchronous worker context.

gpu/ipc/client/command_buffer_proxy_impl.cc

Index: gpu/ipc/client/command_buffer_proxy_impl.cc
diff --git a/gpu/ipc/client/command_buffer_proxy_impl.cc b/gpu/ipc/client/command_buffer_proxy_impl.cc
index daeaa0b8936ade41ca74211a1cd28ca744507a6a..5605f457118bfe0269f08d237616689ae994e195 100644
--- a/gpu/ipc/client/command_buffer_proxy_impl.cc
+++ b/gpu/ipc/client/command_buffer_proxy_impl.cc
@@ -60,7 +60,7 @@ CommandBufferProxyImpl::CommandBufferProxyImpl(
       weak_this_(AsWeakPtr()),
       callback_thread_(base::ThreadTaskRunnerHandle::Get()) {
   DCHECK(channel_);
-  DCHECK(stream_id);
+  DCHECK_NE(stream_id, GPU_STREAM_INVALID);
 }
 
 CommandBufferProxyImpl::~CommandBufferProxyImpl() {
@@ -580,7 +580,7 @@ bool CommandBufferProxyImpl::CanWaitUnverifiedSyncToken(
 
   // If waiting on a different stream, flush pending commands on that stream.
   const int32_t release_stream_id = sync_token->extra_data_field();
-  if (release_stream_id == 0)
+  if (release_stream_id == gpu::GPU_STREAM_INVALID)
     return false;
 
   if (release_stream_id != stream_id_)