Some specifics from bettes/markchang meeting:

* Kill the HTSYSMENU region entirely for the main browser window.  It doesn't really work in the proposed new frame layout (w/new tab button on the left), and it's both undiscoverable and hazardous (data loss) in the absence of a window icon.  It can stay for windows that have icons.
* Plan to have an 8 DIP top drag handle and some kind of post-tabstrip drag region (pkasting suggests starting with 50 DIP), but we should make that 8 DIP top handle controllable under a (separate) command-line flag so we can experiment with omitting it (a la Edge).
* For users who have colored titlebars, we'll continue to make the frame respect the user's active and inactive titlebar colors.  For users who have a colored active titlebar but no colored inactive titlebar (which I think will default to white), we'll use the spec "background tab grey" as the inactive frame color.  For users without colored titlebars, we'll use the spec colors as-is, and indicate window active state by modifying the opacity of the caption 
button icons/tab text in a way that duplicates the native functionality.
* In the single-tab case, we'll draw the "unified white top tabstrip/frame" (no discernable "tab" shape) for active windows only; inactive windows will draw the "inactive" frame color just like in the multi-tab case.  For users without colored titlebars, this means inactive single-tab windows will use a grey frame.

Oh, one other; when transitioning between single-tab and multi-tab mode on the active window, since the frame color will go from white to (whatever the inactive color is, likely grey), we may want to quickly fade from one color to the other instead of just instantly change.  This will likely require experimentation and may be a lower-priority followup.

I would also make the single-tab stuff in general P3.  Let's do single-tab mode after we get any-number-of-tabs mode working.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b6138ca262f9bb35b44d11b7a29ca75b7281a222

commit b6138ca262f9bb35b44d11b7a29ca75b7281a222
Author: Peter Kasting <pkasting@chromium.org>
Date: Tue May 01 19:30:47 2018

Add drag region after tabs.

This also adds some plumbing that will be used later for variable tab
corner radii, and safes a few spots against crashes on startup (found
when testing with unusual mode combinations, e.g. TRAILING NTB in
non-MATERIAL_REFRESH mode).

BUG= 822037 
TEST=none

Change-Id: Iba16ab3f238887c0c93feac67451c36a7317e389
Reviewed-on: https://chromium-review.googlesource.com/1029422
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555145}
[modify] https://crrev.com/b6138ca262f9bb35b44d11b7a29ca75b7281a222/chrome/browser/ui/layout_constants.cc
[modify] https://crrev.com/b6138ca262f9bb35b44d11b7a29ca75b7281a222/chrome/browser/ui/layout_constants.h
[modify] https://crrev.com/b6138ca262f9bb35b44d11b7a29ca75b7281a222/chrome/browser/ui/views/tabs/tab.cc
[modify] https://crrev.com/b6138ca262f9bb35b44d11b7a29ca75b7281a222/chrome/browser/ui/views/tabs/tab.h
[modify] https://crrev.com/b6138ca262f9bb35b44d11b7a29ca75b7281a222/chrome/browser/ui/views/tabs/tab_strip.cc
[modify] https://crrev.com/b6138ca262f9bb35b44d11b7a29ca75b7281a222/chrome/browser/ui/views/tabs/tab_strip.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e557f931c317a2ff3dfdb5501451b83251a3c40f

commit e557f931c317a2ff3dfdb5501451b83251a3c40f
Author: Peter Kasting <pkasting@chromium.org>
Date: Wed May 02 00:26:52 2018

For Refresh, let the tabstrip extend all the way to the caption buttons.

BUG= 822037 
TEST=none

Change-Id: I3c5f0199ea30b9ade417c7209a6be69f629c2ae2
Reviewed-on: https://chromium-review.googlesource.com/1029418
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555249}
[modify] https://crrev.com/e557f931c317a2ff3dfdb5501451b83251a3c40f/chrome/browser/ui/views/frame/browser_non_client_frame_view_ash.cc
[modify] https://crrev.com/e557f931c317a2ff3dfdb5501451b83251a3c40f/chrome/browser/ui/views/frame/browser_non_client_frame_view_mus.cc
[modify] https://crrev.com/e557f931c317a2ff3dfdb5501451b83251a3c40f/chrome/browser/ui/views/frame/glass_browser_frame_view.cc
[modify] https://crrev.com/e557f931c317a2ff3dfdb5501451b83251a3c40f/chrome/browser/ui/views/frame/glass_browser_frame_view.h
[modify] https://crrev.com/e557f931c317a2ff3dfdb5501451b83251a3c40f/chrome/browser/ui/views/frame/opaque_browser_frame_view_layout.cc
[modify] https://crrev.com/e557f931c317a2ff3dfdb5501451b83251a3c40f/chrome/browser/ui/views/frame/opaque_browser_frame_view_layout.h

Reassigning to pkasting@ since there are remaining CLs in flight.

I think the only thing this bug covers that is not covered by other bugs is killing HTSYSMENU; otherwise it's a metabug.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/01fe69c9e42673d400f5bd02bd0f52c8e8cb80fa

commit 01fe69c9e42673d400f5bd02bd0f52c8e8cb80fa
Author: Peter Kasting <pkasting@chromium.org>
Date: Mon Jun 04 22:35:25 2018

Remove HTSYSMENU area from window frame in Refresh mode.

BUG= 822037 
TEST=none

Change-Id: I5f2124a312c55fbf2f65d8d75f33f5189dfbb1c2
Reviewed-on: https://chromium-review.googlesource.com/1049117
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564274}
[modify] https://crrev.com/01fe69c9e42673d400f5bd02bd0f52c8e8cb80fa/chrome/browser/ui/views/frame/glass_browser_frame_view.cc
[modify] https://crrev.com/01fe69c9e42673d400f5bd02bd0f52c8e8cb80fa/chrome/browser/ui/views/frame/opaque_browser_frame_view.cc

Moving meta bugs to PM to make eng bandwidth more clear.