May I work on this?

This is not a good first bug - there's much to consider. Some examples:
 - user themes,
 - NSVisualEffectView usage,
 - cross-platform code overlap (e.g. in ui::NativeTheme),
 - missing titlebar gradients (e.g. compared to safari),
 - missing subpixel AA for the profile switcher,
 - NSFullSizeContentView usage and "adding an unknown subview"
 - Specific code to support 10.9 properly
 - fullscreen behavior

The blocked bug -  issue 671916  - isn't on the roadmap yet.

I see. Thanks :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bfb22fd5548521c3d9796ecfc1817e14151ab5d1

commit bfb22fd5548521c3d9796ecfc1817e14151ab5d1
Author: Elly Fong-Jones <ellyjones@google.com>
Date: Thu Aug 31 18:55:01 2017

views: remove BrowserNonClientFrameViewMac::PaintToolbarBackground

This method is painting the toolbar background incorrectly, and the parent
class's implementation does it correctly.

Bug:  712271 
Change-Id: Ib9fd3ef67799545ec6c826b0e2e9c3a4b3cc3486
Reviewed-on: https://chromium-review.googlesource.com/646451
Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498938}
[modify] https://crrev.com/bfb22fd5548521c3d9796ecfc1817e14151ab5d1/chrome/browser/ui/views/frame/browser_non_client_frame_view_mac.h
[modify] https://crrev.com/bfb22fd5548521c3d9796ecfc1817e14151ab5d1/chrome/browser/ui/views/frame/browser_non_client_frame_view_mac.mm

[Bulk Edit]
Applying M-68 milestone per email discussion with ellyjones@. Pls change it if milestone is incorrectly applied. 

MacViews triage: this was fixed by #5.