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When the device-scale-factor changes, the change is currently handled
in the window server. The server re-generate the local surface id and
notifies to the client through OnWindowBoundsChanged. When this
mojo call arrives to the client, it's propagated to
TopLevelAllocator::OnDeviceScaleFactorChange, and it re-generate again
the local surface id, and sends back to the window server through
SetWindowBounds(). This causes some troubles.

This means that TopLevelAllocator does not have to generate the local
surface id on DSF change since it's already handled in the server.
This CL skips this regeneration.

Bug: 942647
Test: the new test case in aura_unittests
Change-Id: I86f9d8952082f1b2e7bb9ab74e38f556783447f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1540069Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644613}