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When we run web tests with --enable-thread-compositing we have
a compositor thread available.
The code in web_page_popup_impl.cc assumes all web tests are run
single-threaded.
In single-threaded web tests, the owner frame tree runs the
composite step for the popup.
If --enable-thread-compositing is used allow the popup to run
the composite step.

      virtual/scroll-unification/fast/forms/suggestion-picker,
      virtual/scroll-unification/fast/time

Bug: 1144434
TEST: virtual/scroll-unification/fast/forms/select-popup,
Change-Id: I969f825fd6fc331f1e1ed682b98b3e18caeb4f5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636296
Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846249}