Force virtual contexts on Chrome OS ARM

The Mali driver (which is currently the only ARM Chrome OS driver) doesn't
support strict ordering between contexts, so we need virtual contexts.

Currently we can't gather the GL_VENDOR/GL_RENDERER on Chrome OS for use in the
workarounds list, so we enable it on all Chrome OS ARM.

BUG=285292,chrome-os-partner:22688
R=ajwong@chromium.org, zmo@chromium.org

Review URL: https://codereview.chromium.org/46663009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232009 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232009