PageOverlays are not repainted on navigation when threaded compositing is enabled.

Overlays shows on the first page but after any navigation they "disappear"; the overlays are not deleted, they just are not repainted when the contents of the WebView is.

TEST=new layout test that always passes for non-threaded compositing mode and, in threaded compositing mode, passes with the changes in WebViewImpl.cpp but not without them

BUG= 218802

Review URL: https://chromiumcodereview.appspot.com/15740012

git-svn-id: svn://svn.chromium.org/blink/trunk@154434 bbb929c8-8fbe-4397-9dbb-9b2b20218538