Disable Open Tabs favicon sync for low-end devices.

Saves nearly 1mb of memory.
BUG=280909
R=zea@chromium.org

Review URL: https://codereview.chromium.org/24395009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224842 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224842