Add a missing break found by -Wimplicit-fallthrough.

This was added correctly in https://bugs.webkit.org/show_bug.cgi?id=110496 ,
but https://bugs.webkit.org/show_bug.cgi?id=110497 accidentally removed it.

BUG=177475

Review URL: https://codereview.chromium.org/114093003

git-svn-id: svn://svn.chromium.org/blink/trunk@163989 bbb929c8-8fbe-4397-9dbb-9b2b20218538