Enable correct behaviour for deleting regional indicator symbols
on Linux OS

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

BUG=306948

Review URL: https://codereview.chromium.org/27169003

git-svn-id: svn://svn.chromium.org/blink/trunk@160954 bbb929c8-8fbe-4397-9dbb-9b2b20218538