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It turns out layer_tree_host_ can be nullptr in tests which
passes unexpectedly on bots other than UBSan bots. This CL
fixes this behavior.

Bug: 989936
Test: compositor_unittests on the UBsan gn flags
Change-Id: I6e8ff606709b07f19e6309e3b5a96ca240f0de99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1731954Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683419}