CC/GPU: Add a soft limit to the compositor.

On low-end devices (and increasingly high-end) we want to
reduce memory as much as possible for normal content yet
not cause calamitous fallbacks such as raster-on-demand
or flickering.

This adds a hard memory limit in addition to the soft limit.
The hard limit will be used only to avoid calamitous behavior.

Limits are the same for this patch and will be tweaked in
future patches.

BUG=339144
NOTRY=true

No-try since this is cc-only, cc_unittests pass, and the errors are not related.

Review URL: https://codereview.chromium.org/140673009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251180 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251180