Avoid unnecessary CoW for outer DataRef when applying transform-origin

Even if the transform-origin is the same as the current value,
rareNonInheritedData will still be copied.
Add a new macro SET_NESTED_VAR, similar to the existing SET_VAR, but
allowing an intermediate |base| field to be specified, avoiding the
access in the dereference of that field from the group.

BUG=571183

Review URL: https://codereview.chromium.org/1636503005

Cr-Commit-Position: refs/heads/master@{#371766}
1 file changed