Fixed Cr449958/FFmpegRegressionTest.BasicPlayback/0 .  Now OnOpacityChanged may
be called any number of times.  449958 does really switch opacity twice.

ffmpeg regression tests MP4_17 now fails (correctly) with
DEMUXER_ERROR_COULD_NOT_OPEN, rather than PIPELINE_OK

BUG=677211,677214

Review-Url: https://codereview.chromium.org/2639333002
Cr-Commit-Position: refs/heads/master@{#444428}