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Updating to the same value can cause shelf state update. Check
the value before setting.

This is to merge 71. I'll change the impl side for 72.

Bug: 895744
Test: manual. see bug for repro step.
Change-Id: I9aad35098fc120dbfae365ddb79faf6af0069881
Reviewed-on: https://chromium-review.googlesource.com/c/1316590Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605119}