Use SkSrc_Mode in SkCanvasVideoRenderer, to prevent transparent video frames from blending with reu…

media/filters/skcanvas_video_renderer.cc

 // Copyright (c) 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 "media/filters/skcanvas_video_renderer.h"
 
 #include "base/logging.h"
 #include "media/base/video_frame.h"
 #include "media/base/yuv_convert.h"
 #include "third_party/libyuv/include/libyuv.h"
@@ skipping 195 matching lines @@
     return;
   }
 
   // Check if we should convert and update |last_frame_|.
   if (last_frame_.isNull() ||
       video_frame->timestamp() != last_frame_timestamp_) {
     ConvertVideoFrameToBitmap(video_frame, &last_frame_);
     last_frame_timestamp_ = video_frame->timestamp();
   }
 
+  // Use SRC mode so we completely overwrite the buffer (in case we have alpha)
+  // this means we don't need the extra cost of clearing the buffer first.
+  paint.setXfermode(SkXfermode::Create(SkXfermode::kSrc_Mode));
+
   // Paint using |last_frame_|.
   paint.setFilterLevel(SkPaint::kLow_FilterLevel);
   canvas->drawBitmapRect(last_frame_, NULL, dest, &paint);
 }
 
 }  // namespace media