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This is a reland of b2af1539 which was reverted
due to being pointed out by SoM as a cause of an Android compile failure.

Original change's description:
> Fix crash when exiting mirror mode on tablet
>
> Attempting to create ShelfWidget while shelf is in-app is causing
> a crash because ShelfWidget::DelegateView::UpdateOpaqueBackground is
> called before the widget is initialized.
>
> Moving that call from ShelfWidget::DelegateView constructor into
> ShelfWidget::DelegateView::OnWidgetInitialized.
>
> Added a unit test.
>
> Bug: 1021662
> Test: manual, DisplayManagerTest.ExitMirrorModeInTabletMode
> Change-Id: I32a5f91c834d673b109ab518bc2600908d8dea5c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902273
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Reviewed-by: Manu Cornet <manucornet@chromium.org>
> Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#713464}

TBR=oshima@chromium.org

Bug: 1021662
Change-Id: If28233c15d870fca0c9f6818e9031d98276bfe16
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904159
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713547}