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

media/base/pipeline.cc

Index: media/base/pipeline.cc
diff --git a/media/base/pipeline.cc b/media/base/pipeline.cc
index 92f0199bd6a675b10b8956ef2aab8f0e7e73ce38..151d975f2ba85835eb8f547296ab976b438637d4 100644
--- a/media/base/pipeline.cc
+++ b/media/base/pipeline.cc
@@ -258,6 +258,10 @@ void Pipeline::SetClockForTesting(Clock* clock) {
   clock_.reset(clock);
 }
 
+void Pipeline::SetErrorForTesting(PipelineStatus status) {
+  SetError(status);
+}
+
 void Pipeline::SetState(State next_state) {
   if (state_ != kStarted && next_state == kStarted &&
       !creation_time_.is_null()) {