Make Callback::Reset() return a copy to support use-cases where Run() ends up modifying |*this|.

media/filters/video_renderer_base.cc

Index: media/filters/video_renderer_base.cc
diff --git a/media/filters/video_renderer_base.cc b/media/filters/video_renderer_base.cc
index 3fde230540fb90d7529ccba34d2696b441323fe0..a3155cbb2069856594b70881c7b5a5e38fc88f9c 100644
--- a/media/filters/video_renderer_base.cc
+++ b/media/filters/video_renderer_base.cc
@@ -367,7 +367,7 @@ void VideoRendererBase::FrameReady(scoped_refptr<VideoFrame> frame) {
     // A new seek will be requested after this one completes so there is no
     // point trying to collect more frames.
     state_ = kPrerolled;
-    ResetAndRunCB(&seek_cb_, PIPELINE_OK);
+    seek_cb_.ResetAndRun(PIPELINE_OK);
     return;
   }
 
@@ -426,7 +426,7 @@ void VideoRendererBase::FrameReady(scoped_refptr<VideoFrame> frame) {
 
     // ...and we're done seeking!
     DCHECK(!seek_cb_.is_null());
-    ResetAndRunCB(&seek_cb_, PIPELINE_OK);
+    seek_cb_.ResetAndRun(PIPELINE_OK);
 
     base::AutoUnlock ul(lock_);
     paint_cb_.Run();
@@ -457,7 +457,7 @@ void VideoRendererBase::AttemptFlush_Locked() {
   if (!pending_paint_ && !pending_read_) {
     state_ = kFlushed;
     current_frame_ = NULL;
-    ResetAndRunCB(&flush_cb_);
+    flush_cb_.ResetAndRun();
   }
 }