Add a API test for *DeferringViewInWindowChanges as fixed in r202230

Add a API test for *DeferringViewInWindowChanges as fixed in r202230
https://bugs.webkit.org/show_bug.cgi?id=158947

Reviewed by Simon Fraser.

* TestWebKitAPI/TestWebKitAPI.xcodeproj/project.pbxproj:
* TestWebKitAPI/Tests/WebKit2/DeferredViewInWindowStateChange.mm: Added.
(TestWebKitAPI::didFinishLoadForFrame):
(TestWebKitAPI::setPageLoaderClient):
(TestWebKitAPI::TEST):
Add a test ensuring that in-window state changes are deferred when using
the SPI for deferring them, as fixed in r202230.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@202232 268f45cc-cd09-0410-ab3c-d52691b4dbfc