Move Mac specific snapshotting code into its own file
https://bugs.webkit.org/show_bug.cgi?id=114597

Reviewed by Andreas Kling.

Source/WebCore: 

* WebCore.exp.in:
* WebCore.xcodeproj/project.pbxproj:
* bindings/objc/DOM.mm:
(-[DOMRange renderedImageForcingBlackText:]):
* page/Frame.h:
* page/mac/FrameMac.mm:
(WebCore::Frame::nodeImage):
(WebCore::Frame::dragImageForSelection):
* page/mac/FrameSnapshottingMac.h: Added.
* page/mac/FrameSnapshottingMac.mm: Added.
(WebCore::imageFromRect):
(WebCore::selectionImage):
(WebCore::rangeImage):
(WebCore::snapshotDragImage):
* platform/mac/ClipboardMac.mm:
(WebCore::ClipboardMac::dragNSImage):
Move the four Mac specific snapshotting functions in Frame.h into FrameSnapshottingMac.h/mm and turn
them into free standing functions.

Source/WebKit/mac: 

* WebView/WebHTMLView.mm:
(-[WebHTMLView _selectionDraggingImage]):
(-[WebHTMLView selectionImageForcingBlackText:]):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148413 268f45cc-cd09-0410-ab3c-d52691b4dbfc