Bug 778077 - Implement HTMLMediaElement.fastSeek(time). r=cajbir

Implement HTMLMediaElement.fastSeek(), basically by changing all the
MediaDecoderReader::Seek() overrides to not call
MediaDecoderReader::DecodeToTarget(), and have MediaDecoderReader::DecodeSeek()
call DecodeToTarget() if we're doing an accurate (non-fast) seek.

Update gizmo.mp4 to have a keyframe every second, instead of only 1 keyframe at
the start of stream. This makes the unit test I added more useful for mp4...

I pushed most of the seek target clamping logic in MediaDecoder up into
HTMLMediaElement, so that we're clamping in fewer places. Note
MediaDecoderStateMachine::Seek() still sanity checks the seek target.

We have to update the currentTime/MediaDecoder playback position after a seek
completes now, rather than assuming the seek always got it exactly right.

Removed those pesky assertions about seek target lying in the first frame after
seek, since actually sometimes the media doesn't have samples for all streams
after a seek (either due to the media being encoded like that, or because of a
bug in the platform's decoder, not entirely sure).

Green: https://tbpl.mozilla.org/?tree=Try&rev=b028258565e2