Rewrite media::Pipeline state transition machinery and simplify shutdown.

media/base/media_log.cc

Index: media/base/media_log.cc
diff --git a/media/base/media_log.cc b/media/base/media_log.cc
index e47286d002212e846e2cf432faa1192c2bc5ce1e..a44883b204de91840ff52293c9e2f70a2549bd76 100644
--- a/media/base/media_log.cc
+++ b/media/base/media_log.cc
@@ -71,12 +71,10 @@ const char* MediaLog::PipelineStateToString(Pipeline::State state) {
       return "initAudioRenderer";
     case Pipeline::kInitVideoRenderer:
       return "initVideoRenderer";
-    case Pipeline::kPausing:
-      return "pausing";
+    case Pipeline::kInitPrerolling:
+      return "initPrerolling";
     case Pipeline::kSeeking:
       return "seeking";
-    case Pipeline::kFlushing:
-      return "flushing";
     case Pipeline::kStarting:
       return "starting";
     case Pipeline::kStarted: