[iOS] Add didEnterFullscreen / didExitFullscreen callbacks to WKUIDelegatePrivate

[iOS] Add didEnterFullscreen / didExitFullscreen callbacks to WKUIDelegatePrivate
https://bugs.webkit.org/show_bug.cgi?id=139835
<rdar://problem/19312042>

Reviewed by Anders Carlsson.

Add didEnterFullscreen / didExitFullscreen callbacks to WKUIDelegatePrivate
so that they can be implemented in MobileSafari.

* UIProcess/API/APIUIClient.h:
(API::UIClient::didEnterFullscreen):
(API::UIClient::didExitFullscreen):
* UIProcess/API/Cocoa/WKUIDelegatePrivate.h:
* UIProcess/Cocoa/UIDelegate.h:
* UIProcess/Cocoa/UIDelegate.mm:
(WebKit::UIDelegate::setDelegate):
(WebKit::UIDelegate::UIClient::didEnterFullscreen):
(WebKit::UIDelegate::UIClient::didExitFullscreen):
* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::didEnterFullscreen):
(WebKit::WebPageProxy::didExitFullscreen):
* UIProcess/WebPageProxy.h:
* UIProcess/ios/WebVideoFullscreenManagerProxy.mm:
(WebKit::WebVideoFullscreenManagerProxy::didExitFullscreen):
(WebKit::WebVideoFullscreenManagerProxy::didEnterFullscreen):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@177612 268f45cc-cd09-0410-ab3c-d52691b4dbfc