cc: Change Layer::PushProps conditional to say what it means
cc: Change Layer::PushProps conditional to say what it means

This conditional really only applies when property trees are being used,
even though currently impl-side painting and property trees are
synonymous.
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1166613003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#332962}

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