Bug 1276824 (part 1) - Rename gfxContext::ForDrawTarget{,WithTransform}(). r=milan.

The new names Create{,PreservingTransform}OrNull() better communicate that
these functions (a) do object creation, and (b) are fallible.

widget/nsBaseDragService.cpp

@@ -725,7 +725,7 @@ nsBaseDragService::DrawDragForImage(nsIImageLoadingContent* aImageLoader,
     if (!dt || !dt->IsValid())
       return NS_ERROR_FAILURE;
 
-    RefPtr<gfxContext> ctx = gfxContext::ForDrawTarget(dt);
+    RefPtr<gfxContext> ctx = gfxContext::CreateOrNull(dt);
     if (!ctx)
       return NS_ERROR_FAILURE;