Make it easy to test enabling formal delegate protocols

Make it easy to test enabling formal delegate protocols
https://bugs.webkit.org/show_bug.cgi?id=140892

Reviewed by Dan Bernstein.

Add an WK_ENABLE_FORMAL_DELEGATE_PROTOCOLS define that controls whether delegates should be formal protocols.

* Misc/WebDownload.h:
* WebView/WebFrameLoadDelegate.h:
* WebView/WebPolicyDelegate.h:
* WebView/WebResourceLoadDelegate.h:
* WebView/WebUIDelegate.h:
* WebView/WebView.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@179183 268f45cc-cd09-0410-ab3c-d52691b4dbfc