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This reverts commit 2ad3f0c5.

Reason for revert: CL:https://crrev.com/c/2458768 is likely the cause that create ash_unittests and browser_tests failures in linux-chromeos-dbg build starting from https://ci.chromium.org/p/chromium/builders/ci/linux-chromeos-dbg/20815. The failed tests are FrameSizeButtonTest, ImmersiveModeBrowserViewTestNoWebUiTabStrip and ImmersiveModeBrowserViewTest. I'm not 100% sure this CL is depend on the culprit, but it is linked from that CL, so I reverted this just in case. 

Original change's description:
> Add TabletState::Observer and adopt in BrowserNonClientFrameViewAsh
>
> This replaces the use of ash::TabletMode and ash::TabletModeObserver in
> BrowserNonClientFrameViewAsh.
>
> This corresponds to phase 2.6 on the design document [1].
>
> Covered by existing tests, eg
>
> BrowserNonClientFrameViewAshTest.BrowserHeaderVisibilityInTabletModeTest
> HomeLauncherBrowserNonClientFrameViewAshTest.TabletModeBrowserCaptionButtonVisibility
>
> [1] https://docs.google.com/document/d/1xHwcHrAiEaWuA4usGEqKZ9zm1H8SGk3WkS-Jf2Q_los/
>
> BUG=1113900
> R=​sky@chromium.org
>
> Change-Id: I6a21ca44538e52a2712bbb936221aa24063daa36
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462560
> Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
> Reviewed-by: James Cook <jamescook@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#816316}

TBR=jamescook@chromium.org,sky@chromium.org,tonikitoo@igalia.com

Change-Id: I8f8ad38e07c26eee09fec0efc6ca3cbf882a2719
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1113900
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466014Reviewed-by: Maggie Cai <mxcai@chromium.org>
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816384}