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In the case of nested sticky elements, if the descendant elements are promoted
but their ancestor is not we will position them incorrectly. Instead of trying
to re-compute main-thread positioning on the compositor, we can just promote
any sticky element that has promoted sticky descendants.

This CL goes slightly further and promotes any sticky element that has
composited descendants regardless of whether they are sticky - doing so makes
the code much simpler and the performance impact should be limited by the fact
that sticky is not common.

BUG=702229
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2754983002
Cr-Commit-Position: refs/heads/master@{#471217}