Remove obsolete WebFrameClient::didClearWindowObject signature

BUG=none
R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/136493002

git-svn-id: svn://svn.chromium.org/blink/trunk@164960 bbb929c8-8fbe-4397-9dbb-9b2b20218538