Pipeline: Invalidate weak pointers before returning stop callback.

media/base/pipeline_unittest.cc

Index: media/base/pipeline_unittest.cc
diff --git a/media/base/pipeline_unittest.cc b/media/base/pipeline_unittest.cc
index 412fd88bafd836fd51371eaefdf3922d42126ad3..4eb84f62c3f98eaa88a37c0dca1a1674527e546a 100644
--- a/media/base/pipeline_unittest.cc
+++ b/media/base/pipeline_unittest.cc
@@ -315,6 +315,9 @@ class PipelineTest : public ::testing::Test {
   }
 
   void DestroyPipeline() {
+    // In real code Pipeline could be destroyed on a different thread. All weak
+    // pointers must have been invalidated before the stop callback returns.
+    DCHECK(!pipeline_->HasWeakPtrsForTesting());
     pipeline_.reset();
   }