aura: Cache and reuse textures when capturing to a video stream.

When capturing the contents of a tab many times in a row for a
video stream, it is a performance bottleneck to keep creating
and deleting the textures we use.

Since cc now learnt how to use textures that we provide, make
use of that and keep a cache of textures that we are using for
readbacks while a frame subscription is active.

This adds the OwnedMailbox class to allow up to keep a texture
id alive along with its bound mailbox name while passing it
to callbacks.

Depends on: https://codereview.chromium.org/99253004/

R=piman@chromium.org
BUG=312497

Review URL: https://codereview.chromium.org/99203005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238605 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238605