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This CL adds a feature pod button that can toggle RotationLock state.
The button is only visible when tablet mode is enabled.

Screenshot: http://screen/iPZnUeRwsRd

UnifiedSystemTray design doc: go/cros-qs-restyling

TEST=RotationLockFeaturePodControllerTest
TEST=out/Release/chrome --enable-features=SystemTrayUnified --force-tablet-mode=touch_view
BUG=813499

Change-Id: Ia8aff60ea3e0a704024e1402bd6bb52e4f3783c4
Reviewed-on: https://chromium-review.googlesource.com/948223Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541046}