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: Iad2fa90b0bb0687f0151b5a23d1dfb964612d123