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

media/base/composite_filter.cc

Index: media/base/composite_filter.cc
diff --git a/media/base/composite_filter.cc b/media/base/composite_filter.cc
index 20c3ff821e09664795cd2a064f5402b92126bc01..88df408cf9ca09414ffde05e44e60cedcd21c10d 100644
--- a/media/base/composite_filter.cc
+++ b/media/base/composite_filter.cc
@@ -290,14 +290,14 @@ void CompositeFilter::DispatchPendingCallback(PipelineStatus status) {
   DCHECK(status_cb_.is_null() ^ callback_.is_null());
 
   if (!status_cb_.is_null()) {
-    ResetAndRunCB(&status_cb_, status);
+    status_cb_.ResetAndRun(status);
     return;
   }
 
   if (!callback_.is_null()) {
     if (status != PIPELINE_OK)
       SendErrorToHost(status);
-    ResetAndRunCB(&callback_);
+    callback_.ResetAndRun();
   }
 }