Convert another warning after r287092 (missed it in blink r179327)

BUG=none
NOTRY=true

Review URL: https://codereview.chromium.org/439103002

git-svn-id: svn://svn.chromium.org/blink/trunk@179456 bbb929c8-8fbe-4397-9dbb-9b2b20218538