WebKit2 snapshots should contain accelerated content

WebKit2 snapshots should contain accelerated content
<rdar://problem/8807393>
https://bugs.webkit.org/show_bug.cgi?id=51660

Reviewed by Anders Carlsson.

When creating a snapshot, flatten the layer tree for painting so that
accelerated content is included in the snapshot.

* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::snapshotInViewCoordinates):
(WebKit::WebPage::snapshotInDocumentCoordinates):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@74704 268f45cc-cd09-0410-ab3c-d52691b4dbfc