gpu: Add flag for enabling asynchronous worker context.

gpu/ipc/client/gpu_channel_host.h

Index: gpu/ipc/client/gpu_channel_host.h
diff --git a/gpu/ipc/client/gpu_channel_host.h b/gpu/ipc/client/gpu_channel_host.h
index 94407d144e7f8bcdde358ac5e73c16c8dda75283..2efa57d3ac959572e8f96f0f55c8f2a0f98e0135 100644
--- a/gpu/ipc/client/gpu_channel_host.h
+++ b/gpu/ipc/client/gpu_channel_host.h
@@ -78,10 +78,6 @@ class GPU_EXPORT GpuChannelHost
       base::WaitableEvent* shutdown_event,
       gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager);
 
-  static const int32_t kDefaultStreamId = gpu::GPU_STREAM_DEFAULT;
-  static const gpu::GpuStreamPriority kDefaultStreamPriority =
-      gpu::GpuStreamPriority::NORMAL;
-
   bool IsLost() const {
     DCHECK(channel_filter_.get());
     return channel_filter_->IsLost();