Support setting a background color on page overlays

Support setting a background color on page overlays
https://bugs.webkit.org/show_bug.cgi?id=131600

Reviewed by Darin Adler.

* WebProcess/WebPage/PageOverlay.cpp:
(WebKit::PageOverlay::PageOverlay):
(WebKit::PageOverlay::bounds):
(WebKit::PageOverlay::startFadeInAnimation):
(WebKit::PageOverlay::startFadeOutAnimation):
(WebKit::PageOverlay::startFadeAnimation):
(WebKit::PageOverlay::fadeAnimationTimerFired):
Minor style adjustments.
Use more references everywhere.

(WebKit::PageOverlay::setBackgroundColor):
(WebKit::PageOverlay::setNeedsDisplay):
* WebProcess/WebPage/PageOverlay.h:
(WebKit::PageOverlay::backgroundColor):
Keep track of our background color, and push it down to the page overlay controller if we have one.

* WebProcess/WebPage/PageOverlayController.cpp:
(WebKit::updateOverlayGeometry):
(WebKit::PageOverlayController::clearPageOverlay):
Page overlays will always have the right size, and just use drawsContents to determine
whether or not they should tile/have backing store/etc.

(WebKit::PageOverlayController::installPageOverlay):
(WebKit::PageOverlayController::didChangeOverlayBackgroundColor):
Set the background color of the layer.

* WebProcess/WebPage/PageOverlayController.h:

* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::WebPage):
References!

* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::setDocumentOverlayRootLayer):
Mark the compositing tree as needing a rebuild when we get a new document-relative
overlay layer; otherwise we were depending on something else coming along and
requiring a rebuild, which didn't always happen.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@167216 268f45cc-cd09-0410-ab3c-d52691b4dbfc