PDFs still don't snapshot properly in iOS Safari

PDFs still don't snapshot properly in iOS Safari
https://bugs.webkit.org/show_bug.cgi?id=143976
<rdar://problem/18283459>

Reviewed by Anders Carlsson.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::didLayoutForCustomContentProvider):
* UIProcess/WebPageProxy.h:
* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView _didFinishLoadingDataForCustomContentProviderWithSuggestedFilename:data:]):
Inform the client that we've passed all reasonable layout milestones
as soon as the custom content provider has been handed its data.
WKPDFView, the only custom content provider, synchronously lays out
its subviews upon initial receipt of data, so this works fine for it.
This ensures that clients that normally depend on layout milestones firing
won't break when a custom content view is installed.

(-[WKWebView _snapshotRect:intoImageOfWidth:completionHandler:]):
Fall back to renderInContext: if the custom content view is not parented
when a snapshot is requested.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@183068 268f45cc-cd09-0410-ab3c-d52691b4dbfc