Cursor overlaps the text inside New message body in GMail after r174133

Fix regression after r174133 that was causing the cursor to overlap the text
inside "New message" body in GMail. Considering RIGHT-TO-LEFT MARK and
RIGHT-TO-LEFT OVERRIDE in complex text was causing trouble on Mac, not sure of
exact reason.

R=eae@chromium.org
BUG=377445

Review URL: https://codereview.chromium.org/370523003

git-svn-id: svn://svn.chromium.org/blink/trunk@178901 bbb929c8-8fbe-4397-9dbb-9b2b20218538