Allow clients to ask for the WebView/WKView placeholder view when in full screen mode.
https://bugs.webkit.org/show_bug.cgi?id=103558
<rdar://problem/12763112>

Reviewed by Benjamin Poulain.

Source/WebKit/mac:

Clients may want to behave differently when their WebView/WKView has been swapped out by a placeholder
view when in full screen mode.  Add a simple accessor for the existing placeholder view to
WebFullScreenController and WebView.

* WebView/WebFullScreenController.h:
* WebView/WebFullScreenController.mm:
(-[WebFullScreenController webViewPlaceholder]): Added simple accessor.
* WebView/WebView.mm:
(-[WebView fullScreenPlaceholderView]): Added simple accessor.

Source/WebKit2:

Clients may want to behave differently when their WebView/WKView has been swapped out by a placeholder
view when in full screen mode.  Add a simple accessor for the existing placeholder view to
WKFullScreenWindowController and WKView.

* UIProcess/API/mac/WKView.mm:
(-[WKView fullScreenPlaceholderView]): Added simple accessor.
* UIProcess/API/mac/WKViewPrivate.h:
* UIProcess/mac/WKFullScreenWindowController.h:
* UIProcess/mac/WKFullScreenWindowController.mm:
(-[WKFullScreenWindowController webViewPlaceholder]): Added simple accessor.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141173 268f45cc-cd09-0410-ab3c-d52691b4dbfc