Webkit does not build due to unused parameters and -Werror

Some conditional paths in WebKit produce "unused parameter" warnings, which are then treated as errors (WebKit.pri).

Some examples:

WebCore/platform/qt/QWebPageClient.h:

#if USE(ACCELERATED_COMPOSITING)
virtual void setRootGraphicsLayer(WebCore::PlatformLayer* layer) { }
[...]
virtual void markForSync(bool scheduleSync = false) {}

WebCore/accessibility/AXObjectCache.h

#if !HAVE(ACCESSIBILITY)
[...]
inline void AXObjectCache::postNotification(RenderObject*, AXNotification, bool postToElement, PostType) { }
inline void AXObjectCache::postNotification(AccessibilityObject*, Document*, AXNotification, bool postToElement, PostType) { }