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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}