Reland "Repurpose the confirm-quit-bubble to a quit-instruction-bubble"

This is a reland of c463a13bdfe19a4112e14276760e707e0e7d007e

This CL removes BrowserShutdownBrowserTest.ShutdownConfirmation on non-ChromeOS
since the test is only meaningful on ChromeOS.

Original change's description:
> Repurpose the confirm-quit-bubble to a quit-instruction-bubble
>
> It was decided in [1] that the Ctrl+Shift+Q shortcut should be removed
> altogether and replaced with a bubble giving an alternative way to quit using
> the keyboard (similar to the "Press Alt+Left to go back" bubble).
>
> [1] https://docs.google.com/document/d/1mvTBrxD58GIsqFiqgiLCZiXy1iOtDpQx_Tp5OSuYWEU/edit?usp=sharing
>
> BUG=243164
> R=sky
>
> Change-Id: I47578d6b750c45fbc5d0a6e6ffe7e5f528dbe80d
> Reviewed-on: https://chromium-review.googlesource.com/1173654
> Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#583138}

TBR=sky

Bug: 243164
Change-Id: I376e9b31c071b0c08e7f5cbe9e2bbda22ee1f1ac
Reviewed-on: https://chromium-review.googlesource.com/1178165
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583781}
26 files changed