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This change fixes several issues that were causing crashes.
- MediaPlayerHostMsg_DemuxerReady_Params was not initializing the xxx_codec
member variables so audio-only or video-only content would randomly be
flagged as A/V content with a random codec ID.
- ChunkDemuxer::Stop() was not being called which could cause the delegate
to be accessed after it was destroyed.
- MediaSourceDelegate was signalling that it had metadata before the 
ChunkDemuxer was even initialized. This caused the HTMLMediaElement to
signal that the load completed way too early and caused play() to be
called when the delegate wasn't ready.

BUG=233420
TEST=webkitmediasource-play.html LayoutTest reliably runs w/o crashing now.

Review URL: https://chromiumcodereview.appspot.com/15898002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@202019 0039d316-1c4b-4281-b951-d872f2087c98