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 b4c256846a9912ce99d5fad96f75a7f6ab52406a..9e4d54320a6923f9b48485e8c54a46b554403fad 100644
--- a/media/filters/video_renderer_base.cc
+++ b/media/filters/video_renderer_base.cc
@@ -362,7 +362,7 @@ 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;
   }
 
@@ -421,7 +421,7 @@ 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();
@@ -452,7 +452,7 @@ void VideoRendererBase::AttemptFlush_Locked() {
   if (!pending_paint_ && !pending_read_) {
     state_ = kFlushed;
     current_frame_ = NULL;
-    ResetAndRunCB(&flush_cb_);
+    flush_cb_.ResetAndRun();
   }
 }