Adds RootWindowHost::Create() for android

Implementation CHECKs, so this is only needed for tests and ash, which
we're not worrying about yet.

BUG=none
TEST=none
R=ben@chromium.org

Review URL: https://codereview.chromium.org/128743002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243711 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243711