Indicate in which world the window object was cleared

Otherwise, an embedder gets multiple calls for the same frame and has no
way to distinguish which world the window was cleared in.

BUG=none
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/136023002

git-svn-id: svn://svn.chromium.org/blink/trunk@164947 bbb929c8-8fbe-4397-9dbb-9b2b20218538