Add helper macro for forward declaring objective-c classes

Add helper macro for forward declaring objective-c classes
https://bugs.webkit.org/show_bug.cgi?id=76485

Reviewed by Anders Carlsson.

Source/JavaScriptCore:

* wtf/Compiler.h:
Add OBJC_CLASS macro which helps reduce code when forward declaring an
objective-c class in a header which can be included from both Objective-C
and non-Objective-C files.

Source/WebCore:

* accessibility/AccessibilityObject.h:
* bindings/js/ScriptController.h:
* bridge/objc/objc_utility.h:
* page/DragClient.h:
* page/EditorClient.h:
* platform/AutodrainedPool.h:
* platform/ContextMenuItem.h:
* platform/Cursor.h:
* platform/DragData.h:
* platform/DragImage.h:
* platform/KURL.h:
* platform/Pasteboard.h:
* platform/PlatformKeyboardEvent.h:
* platform/PlatformMenuDescription.h:
* platform/PlatformScreen.h:
* platform/SharedBuffer.h:
* platform/Widget.h:
* platform/cf/SchedulePair.h:
* platform/graphics/BitmapImage.h:
* platform/graphics/FontPlatformData.h:
* platform/graphics/GraphicsContext3D.h:
* platform/graphics/GraphicsLayer.h:
* platform/graphics/Icon.h:
* platform/graphics/Image.h:
* platform/graphics/MediaPlayer.h:
* platform/graphics/avfoundation/objc/MediaPlayerPrivateAVFoundationObjC.h:
* platform/graphics/ca/PlatformCAAnimation.h:
* platform/graphics/ca/mac/TileCache.h:
* platform/graphics/mac/ColorMac.h:
* platform/graphics/mac/MediaPlayerPrivateQTKit.h:
* platform/graphics/mac/MediaPlayerProxy.h:
* platform/mac/ClipboardMac.h:
* platform/mac/LocalCurrentGraphicsContext.h:
* platform/mac/PasteboardHelper.h:
* platform/mac/PopupMenuMac.h:
* platform/mac/ScrollAnimatorMac.h:
* platform/mac/WebCoreSystemInterface.h:
* platform/network/ResourceHandle.h:
* platform/network/ResourceHandleClient.h:
* platform/network/ResourceHandleInternal.h:
* platform/network/cf/AuthenticationChallenge.h:
* platform/network/cf/ResourceError.h:
* platform/network/cf/ResourceRequest.h:
* platform/network/cf/ResourceResponse.h:
* rendering/RenderThemeMac.h:
Deploy OBJC_CLASS for a little code reduction.

Source/WebKit2:

* WebProcess/WebCoreSupport/WebDragClient.h:
Deploy OBJC_CLASS for a little code reduction.

* config.h:
Remove duplicate copy of OBJC_CLASS.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105203 268f45cc-cd09-0410-ab3c-d52691b4dbfc