[Android] Add a low-end device mode override flag

The current low-end device optimizations are restricted to devices that meet
certain memory and OS version criteria.  Add a flag that overrides this
detection, simplifying testing and allowing power users to take advantage
of the optimizations.

BUG=320747

Review URL: https://codereview.chromium.org/62833002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237584 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237584