cc: Return a NullCanvas instead of NULL in context lost situations.

cc/resources/tile_manager.cc

 // Copyright 2012 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/resources/tile_manager.h"
 
 #include <algorithm>
 #include <limits>
 #include <string>
 
@@ skipping 105 matching lines @@
     frame_viewer_instrumentation::ScopedRasterTask raster_task(
         tile_id_,
         tile_resolution_,
         source_frame_number_,
         layer_id_,
         raster_mode_);
     devtools_instrumentation::ScopedLayerTask layer_task(
         devtools_instrumentation::kRasterTask, layer_id_);
 
     skia::RefPtr<SkCanvas> canvas = raster_buffer_->AcquireSkCanvas();
-    if (!canvas)
-      return;
-    canvas->save();
+    DCHECK(canvas);
 
     skia::RefPtr<SkDrawFilter> draw_filter;
     switch (raster_mode_) {
       case LOW_QUALITY_RASTER_MODE:
         draw_filter = skia::AdoptRef(new skia::PaintSimplifier);
         break;
       case HIGH_QUALITY_RASTER_MODE:
         break;
       case NUM_RASTER_MODES:
       default:
@@ skipping 18 matching lines @@
       base::TimeDelta current_rasterize_time =
           rendering_stats_->impl_thread_rendering_stats().rasterize_time;
       LOCAL_HISTOGRAM_CUSTOM_COUNTS(
           "Renderer4.PictureRasterTimeUS",
           (current_rasterize_time - prev_rasterize_time).InMicroseconds(),
           0,
           100000,
           100);
     }
 
-    canvas->restore();
     raster_buffer_->ReleaseSkCanvas(canvas);
   }
 
   PicturePileImpl::Analysis analysis_;
   scoped_refptr<PicturePileImpl> picture_pile_;
   gfx::Rect content_rect_;
   float contents_scale_;
   RasterMode raster_mode_;
   TileResolution tile_resolution_;
   int layer_id_;
@@ skipping 993 matching lines @@
   TRACE_EVENT0("cc", "TileManager::CheckIfReadyToActivate");
 
   rasterizer_->CheckForCompletedTasks();
   did_check_for_completed_tasks_since_last_schedule_tasks_ = true;
 
   if (IsReadyToActivate())
     client_->NotifyReadyToActivate();
 }
 
 }  // namespace cc