Revert of Remove media::VideoRenderer::SetPlaybackRate(). (https://codereview.chromiu...

Revert of Remove media::VideoRenderer::SetPlaybackRate(). (https://codereview.chromium.org/384943002/)

Reason for revert:
Sorry to revert this change. It made the following layout tests flakily fail:
media/track/track-cue-rendering-horizontal.html
media/track/track-cue-rendering-vertical.html

Reverting locally healed the tests.

The image diffs show what appears to be a timestamp change (something like 0:18 to 0:19):
https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Linux__dbg_/20779/layout-test-results/results.html

Original issue's description:
> Remove media::VideoRenderer::SetPlaybackRate().
>
> Video renderers don't need the information as they are already making
> audio/video synchronization decisions based on a media timeline that
> already incorporates the current playback rate via the time callback.
>
> In particular, VideoRendererImpl only used playback rate to estimate
> a better duration to sleep until the current frame was ready ...
> except that in most cases we'd sleep for kIdleTimeDelta and wait
> until the media timeline had progressed past the current frame's
> timestamp. In other words, there's absolutely no reason to even try
> to estimate the sleep duration based on the playback rate.
>
> BUG=370634
>
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282797

TBR=xhwang@chromium.org,scherkus@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=370634

Review URL: https://codereview.chromium.org/390733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282891 0039d316-1c4b-4281-b951-d872f2087c98