Re-land: cc: Use worker context for one-copy tile initialization.

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 aac2bd37fabc0f235a84f4199c85fcfbfc631cc9..c070390e2c4e3fd1ab7b9a7b3d65ee24e1aa912a 100644
--- a/cc/raster/gpu_tile_task_worker_pool.cc
+++ b/cc/raster/gpu_tile_task_worker_pool.cc
@@ -43,13 +43,9 @@ class RasterBufferImpl : public RasterBuffer {
     ContextProvider* context_provider = rasterizer_->resource_provider()
                                             ->output_surface()
                                             ->worker_context_provider();
+    DCHECK(context_provider);
 
-    // The context lock must be held while accessing the context on a
-    // worker thread.
-    base::AutoLock context_lock(*context_provider->GetLock());
-
-    // Allow this worker thread to bind to context_provider.
-    context_provider->DetachFromThread();
+    ContextProvider::ScopedContextLock scoped_context(context_provider);
 
     gfx::Rect playback_rect = raster_full_rect;
     if (resource_has_previous_content_) {
@@ -64,10 +60,7 @@ class RasterBufferImpl : public RasterBuffer {
                                  playback_rect, scale);
 
     // Barrier to sync worker context output to cc context.
-    context_provider->ContextGL()->OrderingBarrierCHROMIUM();
-
-    // Allow compositor thread to bind to context_provider.
-    context_provider->DetachFromThread();
+    scoped_context.ContextGL()->OrderingBarrierCHROMIUM();
   }
 
  private: