[Files.app] Support RTL languages in the main window.

BUG=226943
TEST=manually tested
R=hirono@chromium.org

Review URL: https://codereview.chromium.org/26432004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227859 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227859