[Cocoa] Notify the client when a navigation snapshot is taken

[Cocoa] Notify the client when a navigation snapshot is taken
https://bugs.webkit.org/show_bug.cgi?id=134865

Reviewed by Sam Weinig.

* UIProcess/API/Cocoa/WKNavigationDelegatePrivate.h: Declared new delegate method.

* UIProcess/Cocoa/NavigationState.h:
* UIProcess/Cocoa/NavigationState.mm:
(WebKit::NavigationState::setNavigationDelegate): Initialize new flag in
m_navigationDelegateMethods.
(WebKit::NavigationState::willRecordNavigationSnapshot): Added. Calls the new
WKNavigationDelegate method.

* UIProcess/PageClient.h: Declared new client function.

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::willRecordNavigationSnapshot): Added. Calls the new client function.
* UIProcess/WebPageProxy.h:

* UIProcess/ios/PageClientImplIOS.h:
* UIProcess/ios/PageClientImplIOS.mm:
(WebKit::PageClientImpl::willRecordNavigationSnapshot): Override that calls
NavigationState::willRecordNavigationSnapshot.

* UIProcess/mac/PageClientImpl.h:
* UIProcess/mac/PageClientImpl.mm:
(WebKit::PageClientImpl::willRecordNavigationSnapshot): Ditto.

* UIProcess/mac/ViewSnapshotStore.mm:
(WebKit::ViewSnapshotStore::recordSnapshot): Added a call to
WebPageProxy::willRecordNavigationSnapshot.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@171045 268f45cc-cd09-0410-ab3c-d52691b4dbfc