[Cocoa] Add SPI to check if a WKWebView has an inspector frontend

[Cocoa] Add SPI to check if a WKWebView has an inspector frontend
https://bugs.webkit.org/show_bug.cgi?id=193162
<rdar://problem/24267143>

Patch by Joseph Pecoraro <pecoraro@apple.com> on 2019-01-07
Reviewed by Alex Christensen.

* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView _hasInspectorFrontend]):
* UIProcess/API/Cocoa/WKWebViewPrivate.h:
* UIProcess/WebPageProxy.h:
(WebKit::WebPageProxy::hasInspectorFrontend const):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@239691 268f45cc-cd09-0410-ab3c-d52691b4dbfc