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Initializing a decoder is asynchronous, so add a callback to tell client
when the decoder is ready to use.
I confirmed that this works locally using a C plugin that I wrote on my machine.

BUG=NONE
TEST=NONE

Review URL: http://codereview.chromium.org/7065010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@86699 0039d316-1c4b-4281-b951-d872f2087c98