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This changes the behavior of autotestPrivate.setTabletModeEnabled;
it will ensure entering into the specified mode, and will not
end and will not be affected by peripherals.

As is described in the bug, it is crucial to ensure into a certain
mode for conducting an integration test or a performance test, but
sometimes the test scenarios or existence of peripherals can affect
the intermediate states, which may cause failures when combined
with multiple tests. This CL simplifies this situation by adding
a new method to ash::TabletMode.

Note that we can't change the behavior of existing SetEnabledForTest,
because it is widely used by unittests and browsertests.

Bug: 1040292
Test: tast run ui.ScreenRotationPerf ui.WindowResizePerf on scarlet
Change-Id: I343e2b86c145bae789629df1baaf503fa2dfdef5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2016193Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735676}