[webkit-reviews] review requested: [Bug 137164] Move PageOverlay[Controller] to WebCore : [Attachment 239096] add some null checks

[bugzilla-daemon at webkit.org]

Tim Horton <thorton at apple.com> has asked  for review:
Bug 137164: Move PageOverlay[Controller] to WebCore
https://bugs.webkit.org/show_bug.cgi?id=137164

Attachment 239096: add some null checks
https://bugs.webkit.org/attachment.cgi?id=239096&action=review

------- Additional Comments from Tim Horton <thorton at apple.com>
Found some issues while writing tests, adjusting the patch slightly to account
for them (PageOverlayController::installPageOverlay, ::deviceScaleFactor, and
::notifyFlushRequired). Updated changelog as well.