Add a missing break. Found by PVS-Studio and -Wimplicit-fallthrough.

BUG=324935,177475
NOTRY=true

Review URL: https://codereview.chromium.org/99723003

git-svn-id: svn://svn.chromium.org/blink/trunk@163504 bbb929c8-8fbe-4397-9dbb-9b2b20218538