If we set a transform in push_stacking_context, it changes the internal
WebRender behaviour to make that stacking context a reference frame, and
things inside it are positioned differently. This is true even if the
transform is an identity transform.
In most cases we are hitting this and sending an identity transform
through, when in fact we want to be sending a None value to WebRender so
that it doesn't create reference frames. This is a partial fix, a proper
fix will be done in bug 1345577 by separating the CSS transform from the
other transforms that FrameLayerBuilder invents.
MozReview-Commit-ID: ElSs3hFMD2D
0b60039322