Remove references to blink::WebPageOverlay.

This interface no longer exists; overlays are only implemented within Blink
and are no longer part of the public API.

(Since https://codereview.chromium.org/1264483002)

TBR=rbyers@chromium.org

Review-Url: https://codereview.chromium.org/2144853002
Cr-Commit-Position: refs/heads/master@{#404967}
3 files changed