[SPv2] Move PaintPropertyTreeBuilder state for containing blocks into a struct.

This makes it easier to see that copies of all relevant fields are correct,
and also makes it easier to add new state inherited on the containing block chain.

BUG=563667

Review-Url: https://codereview.chromium.org/2144733008
Cr-Commit-Position: refs/heads/master@{#405566}
2 files changed