Mac: Fix flickering while entering/leaving fullscreen low power video

Mac: Fix flickering while entering/leaving fullscreen low power video

Change the low power transition to be 3-state. The states are
- not in FSLP mode, FSLP window is hidden
- warming up FSLP mode, FSLP window is visible and in the fullscreen
  state, but is behind the main window
- in FSLP mode. the FLSP window is in front of the main window

From my experiments, adding up this "warming up" state, makes it so
that the flahses when entering FSLP mode go away.

When exiting FSLP mode, it used to be that we would call
-[AVSampleBufferDisplayLayer flushAndRemoveImage] immediately, since
it never caused any issues. I was able to see some black flashes on
Sierra. If I change it so that we wait for 15 frames to do this, then
the flashes go away.

BUG=661691
Review-Url: https://codereview.chromium.org/2468063004
Cr-Commit-Position: refs/heads/master@{#429515}
(cherry picked from commit d8ca506c965aa6331149055e421c9d2e9c2b14ab)

[ccameron, 2016-11-04 22:05:38]

Committed patchset #1 (id:1) to pending queue manually as
3f61bd2bee9f80e8b5f31a7b006916a94da11178.