cc: Implement shared worker contexts. (v1)

cc/raster/gpu_tile_task_worker_pool.cc

Index: cc/raster/gpu_tile_task_worker_pool.cc
diff --git a/cc/raster/gpu_tile_task_worker_pool.cc b/cc/raster/gpu_tile_task_worker_pool.cc
index 0f3d6077813207b1eaed9d80dcc52e1e3268ba50..dfafec2574d751642a29160c284a51b3b33eba0d 100644
--- a/cc/raster/gpu_tile_task_worker_pool.cc
+++ b/cc/raster/gpu_tile_task_worker_pool.cc
@@ -50,6 +50,10 @@ class RasterBufferImpl : public RasterBuffer {
 
     ContextProvider::ScopedContextLock scoped_context(context_provider);
 
+    gpu::gles2::GLES2Interface* gl = scoped_context.ContextGL();
+    if (!gl)
+      return;
+
     gfx::Rect playback_rect = raster_full_rect;
     if (resource_has_previous_content_) {
       playback_rect.Intersect(raster_dirty_rect);
@@ -63,7 +67,7 @@ class RasterBufferImpl : public RasterBuffer {
                                  playback_rect, scale);
 
     // Barrier to sync worker context output to cc context.
-    scoped_context.ContextGL()->OrderingBarrierCHROMIUM();
+    gl->OrderingBarrierCHROMIUM();
   }
 
  private: