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This fixes OOM crashes in the mediaserver process on older devices
by suspending players more frequently and preventing unused players
and SurfaceTextures from ever being created. Both fixes are required
to enable playback on JellyBean devices; independently neither is
sufficient for playback to succeed.

- On low end devices, suspends all idle players immediately upon
  playback of another player.
- On low end devices, if a codec already exists, codec creation is
  deferred until the first Decode() call. Resolves issues with sites
  like vimeo.com that are appending initialization segments to MSE
  but never any further data.
- Once more than 2 players exist on a low end device, every extra
  player will cause immediate idle player collection. Normal devices
  will now do this after 8 players.

A followup CL will set preload=none for all JellyBean devices.

BUG=612909
TEST=manual, new tests.

Review-Url: https://codereview.chromium.org/2333983002
Cr-Commit-Position: refs/heads/master@{#419375}