Add WKNavigationDelegate SPI to tell the client when an insecure plug-in is blocked to match existing functionality in WKPageLoaderClient. WKPageLoaderClient has a pluginDidFail callback that's invoked when a plug-in fails to initialize and when an insecure plug-in is blocked, but WKNavigationDelegate only defines a -_webView:didFailToInitializePlugInWithInfo: method for the former case.
<rdar://problem/37279560>
Created attachment 333209 [details] Patch
Committed r228194: <https://trac.webkit.org/changeset/228194>