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Now that BlinkGenPropertyTrees has launched, there is no need to track
is_root_for_isolated_group on cc::Layer as it is now tracked using
render surface reasons.

Bug: 993936
Change-Id: Ifa5b0b497392d8cb4020826139a6270c2faaf20e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815610
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698552}