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}
26 files changed