Blink-side changes to make https://codereview.chromium.org/655273005/ possible.

BUG=404710

Review URL: https://codereview.chromium.org/679053003

git-svn-id: svn://svn.chromium.org/blink/trunk@184547 bbb929c8-8fbe-4397-9dbb-9b2b20218538