Enable correct behaviour for deleting regional indicator symbols on Android OS

Regional indicator symbols that are combined should behave as a single
character when deleting.

BUG=313656

Review URL: https://codereview.chromium.org/50293006

git-svn-id: svn://svn.chromium.org/blink/trunk@161191 bbb929c8-8fbe-4397-9dbb-9b2b20218538