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 a5a149b2e90420ff04ffc1718f5af4877af8d89b..f990f0b9b8158c128913fde05f533956171e0afc 100644
--- a/media/base/mock_filters.h
+++ b/media/base/mock_filters.h
@@ -274,11 +274,6 @@ 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();
 }