Renderers don't read from disabled demuxer streams, thus leaving
demuxer stream read position unchanged. But MSE garbage collection
currently stops removing data from the front of buffered ranges when
it reaches the last read position. This results in QuotaExceeded
exceptions being thrown when more data is appended to the disabled
stream. The fix is to seek the disabled stream before running MSE
GC, this will update the read position to the current media_time
and should allow the GC algorithm to remove data as expected.

BUG=713423

Review-Url: https://codereview.chromium.org/2827983004
Cr-Commit-Position: refs/heads/master@{#466136}