Add SPI to expose width and height anchors for WKWebView's content view

Add SPI to expose width and height anchors for WKWebView's content view
https://bugs.webkit.org/show_bug.cgi?id=183711
<rdar://problem/38562899>

Reviewed by Tim Horton.

Source/WebKit:

Add _contentWidthAnchor and _contentHeightAnchor SPI to WKWebView for internal clients to be able to reason
about the size of the content view on iOS using autolayout.

Test: WebKit.AutoLayoutPositionHeaderAndFooterViewsInScrollView.

* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView _contentWidthAnchor]):
(-[WKWebView _contentHeightAnchor]):
* UIProcess/API/Cocoa/WKWebViewPrivate.h:

Tools:

Adds an API test exercising the new SPI.

* TestWebKitAPI/Tests/WebKitCocoa/AutoLayoutIntegration.mm:
(checkCGRect):
(TEST):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@229688 268f45cc-cd09-0410-ab3c-d52691b4dbfc