Re-enable GPU partial raster for non-MSAA content

cc/raster/gpu_raster_buffer_provider.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/raster/gpu_raster_buffer_provider.h"
 
 #include <stdint.h>
 
 #include <algorithm>
 
@@ skipping 208 matching lines @@
   return resource_provider_->best_render_buffer_format();
 }
 
 bool GpuRasterBufferProvider::IsResourceSwizzleRequired(
     bool must_support_alpha) const {
   // This doesn't require a swizzle because we rasterize to the correct format.
   return false;
 }
 
 bool GpuRasterBufferProvider::CanPartialRasterIntoProvidedResource() const {
-  return false;
+  // Partial raster doesn't support MSAA, as the MSAA resolve is unaware of clip
+  // rects.
+  // TODO(crbug.com/629683): See if we can work around this limitation.
+  return msaa_sample_count_ == 0;
 }
 
 void GpuRasterBufferProvider::Shutdown() {
   pending_raster_buffers_.clear();
 }
 
 void GpuRasterBufferProvider::PlaybackOnWorkerThread(
     ResourceProvider::ScopedWriteLockGL* resource_lock,
     const gpu::SyncToken& sync_token,
     bool resource_has_previous_content,
@@ skipping 37 matching lines @@
   gl->OrderingBarrierCHROMIUM();
 
   // Generate sync token after the barrier for cross context synchronization.
   gpu::SyncToken resource_sync_token;
   gl->GenUnverifiedSyncTokenCHROMIUM(fence_sync, resource_sync_token.GetData());
   resource_lock->set_sync_token(resource_sync_token);
   resource_lock->set_synchronized(!async_worker_context_enabled_);
 }
 
 }  // namespace cc