[Cocoa] Report the first paint after committing a load as a rendering progress event

[Cocoa] Report the first paint after committing a load as a rendering progress event
https://bugs.webkit.org/show_bug.cgi?id=146765

Reviewed by Tim Horton.

* Shared/API/Cocoa/_WKRenderingProgressEvents.h: Added _WKRenderingProgressEventFirstPaint.

* UIProcess/API/Cocoa/WKWebView.mm:
(-[WKWebView _didCommitLayerTree:]): If this is the first layer tree commit since the load
was committed, and the delegate is observing _WKRenderingProgressEventFirstPaint, call
NavigationState::didFirstPaint.

* UIProcess/Cocoa/NavigationState.h:
* UIProcess/Cocoa/NavigationState.mm:
(WebKit::NavigationState::didFirstPaint): Added. Calls -_webView:renderingProgressDidChange:
with _WKRenderingProgressEventFirstPaint.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@186586 268f45cc-cd09-0410-ab3c-d52691b4dbfc