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There's a bug where when you activate a pending tree, the active
tree does not get the test_properties(), so if you BuildLayerList on
the active tree, the layer list drops all its layers except the root.

In fixing this, I struggled with understanding where and when tests are
building layer lists and property trees, and made some changes.

Now the test properties build the layer list when AddChild/RemoveChild
happens, which is consistent with SetRootLayerForTesting.

Also, the BuildLayerListAndPropertyTreesForTesting() and (the new)
BuildPropertyTreesForTesting() methods will now always build the trees
by setting needs_rebuild = true internally.

Also, cleaned up a bunch of LayerTreeHostImpl tests, to not have special
PrepareToDraw paths that build property trees, and just do it in the
tests.

R=ajuma, jaydasika@chromium.org
BUG=311404
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2106273002
Cr-Commit-Position: refs/heads/master@{#403375}