Bug 1498639 - Give WR the id of the scroll frame perspective scrolls relative to, and compute the right transform based on that. r=kats,kvark

I think this is as clean as it can get.

Differential Revision: https://phabricator.services.mozilla.com/D17848

gfx/layers/wr/StackingContextHelper.cpp

@@ -36,7 +36,7 @@ StackingContextHelper::StackingContextHelper(
   gfx::Matrix transform2d;
   if (aParams.mBoundTransform &&
       aParams.mBoundTransform->CanDraw2D(&transform2d) &&
-      aParams.reference_frame_kind != wr::ReferenceFrameKind::Perspective &&
+      aParams.reference_frame_kind != wr::WrReferenceFrameKind::Perspective &&
       !aParentSC.mIsPreserve3D) {
     mInheritedTransform = transform2d * aParentSC.mInheritedTransform;