Combine clips in content target space.

We currently combine clips in the target space of the clip's transform
node. This is incorrect. We want to combine clips in the space of the
transform node's _content_ target space.

BUG=386810

Review URL: https://codereview.chromium.org/991013002

Cr-Commit-Position: refs/heads/master@{#319789}
2 files changed