Allow simultaneous audio and video config change

When completing prefetch for multiple streams, if more than one stream has
|kConfigChanged| next, then the calls to DecodeMoreAudio() and
DecodeMoreVideo() need to allow possibility that
|CONFIG_CHANGE_EVENT_PENDING| is already set by the other stream.
Also includes a new unit test for this scenario.

R=qinmin@chromium.org,acolwell@chromium.org,xhwang@chromium.org
BUG=314170
TEST=All media_unittests pass on Android with MediaCodecBridge available.

Review URL: https://codereview.chromium.org/50433007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232920 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232920