Revert FFmpegDemuxer to pre-r171883 capacity behaviour.
Revert FFmpegDemuxer to pre-r171883 capacity behaviour.

The proper fix for improving seek performance is to plumb read cancelation support through our network stack. Unfortunately that's a much larger problem I'm not going to tackle this very moment.

BUG=165757


Review URL: https://chromiumcodereview.appspot.com/11553034

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@172781 0039d316-1c4b-4281-b951-d872f2087c98

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