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Thus only transformed elements now paint with transform. The concept
of painting fixed-position elements with transform was introduced in
https://codereview.chromium.org/1110653003, to help with paint invalidation
of SPv1. However we ended up invalidating whole subtrees on scroll in v1,
so this is not necessary.

Also added invalidation of paint properties when changing compositing mode,
since this can affect whether paintsWithTransform changes its behavior.

BUG=674623
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2570423003
Cr-Commit-Position: refs/heads/master@{#440906}