Add a lot of Pipeline tests to cover stopping/error handling while in a variety of states.

media/base/mock_filters.h

Index: media/base/mock_filters.h
diff --git a/media/base/mock_filters.h b/media/base/mock_filters.h
index 515cd442e90d568da5296c7ec5b06f226314deef..3a87ce5d67c59648d06d55063b17d48a57de7000 100644
--- a/media/base/mock_filters.h
+++ b/media/base/mock_filters.h
@@ -276,11 +276,6 @@ class MockFilterCollection {
   DISALLOW_COPY_AND_ASSIGN(MockFilterCollection);
 };
 
-// Helper gmock action that calls SetError() on behalf of the provided filter.
-ACTION_P2(SetError, filter, error) {
-  filter->host()->SetError(error);
-}
-
 ACTION(RunClosure) {
   arg0.Run();
 }