Merge changes Ie3033144,Ib4008117 into nyc-dev
am: 6fae62de61

* commit '6fae62de61f2315ed43935682b59fe8a5d936cef':
  WebmWriter: respect track add order
  WebmWriter: do not crash on missing format keys

Change-Id: I869c7ab08db4c9e9600d6504f94e3419d628b29e