gpu: Add flag for enabling asynchronous worker context.

content/renderer/render_thread_impl.h

Index: content/renderer/render_thread_impl.h
diff --git a/content/renderer/render_thread_impl.h b/content/renderer/render_thread_impl.h
index 806d32bfbd261725bf64de15bf2db7ae1358d6c4..015cc7daf60c404064c99ed6e7773740d3192fd9 100644
--- a/content/renderer/render_thread_impl.h
+++ b/content/renderer/render_thread_impl.h
@@ -196,6 +196,7 @@ class CONTENT_EXPORT RenderThreadImpl
   // CompositorDependencies implementation.
   bool IsGpuRasterizationForced() override;
   bool IsGpuRasterizationEnabled() override;
+  bool IsAsyncWorkerContextEnabled() override;
   int GetGpuRasterizationMSAASampleCount() override;
   bool IsLcdTextEnabled() override;
   bool IsDistanceFieldTextEnabled() override;
@@ -654,6 +655,7 @@ class CONTENT_EXPORT RenderThreadImpl
   // Compositor settings.
   bool is_gpu_rasterization_enabled_;
   bool is_gpu_rasterization_forced_;
+  bool is_async_worker_context_enabled_;
   int gpu_rasterization_msaa_sample_count_;
   bool is_lcd_text_enabled_;
   bool is_distance_field_text_enabled_;