Avoid comparison of AtomicStrings when they don't have the same impl

When comparing two atomic strings, there's no point in comparing they're content if their StringImpl instances are not the same one.
This CL adds a check to avoid that content comparison in these cases.

BUG=

Review URL: https://codereview.chromium.org/190043003

git-svn-id: svn://svn.chromium.org/blink/trunk@168718 bbb929c8-8fbe-4397-9dbb-9b2b20218538