Doesn't need to block release, but adding a milestone to give it a bit more priority than the other `CodeHealth` ones.

(I think I have an idea what needs to be nuked - should be easier now that the flags are consolidated)

We don't have any gradients left and this bug is obsolete.

We need to remove some dead code still. Things like NativeThemeMac::GetButtonBackgroundShader()

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9bc80e246d791df3e093bf2b98986e7fab36934a

commit 9bc80e246d791df3e093bf2b98986e7fab36934a
Author: ellyjones <ellyjones@chromium.org>
Date: Fri Apr 21 17:27:27 2017

views: remove PlatformStyle combobox hooks

These are no longer needed since views on Mac is Harmony-only.

BUG= 629958 

Review-Url: https://codereview.chromium.org/2826313006
Cr-Commit-Position: refs/heads/master@{#466377}

[modify] https://crrev.com/9bc80e246d791df3e093bf2b98986e7fab36934a/ui/views/BUILD.gn
[modify] https://crrev.com/9bc80e246d791df3e093bf2b98986e7fab36934a/ui/views/controls/combobox/combobox.cc
[modify] https://crrev.com/9bc80e246d791df3e093bf2b98986e7fab36934a/ui/views/controls/combobox/combobox.h
[modify] https://crrev.com/9bc80e246d791df3e093bf2b98986e7fab36934a/ui/views/style/platform_style.cc
[modify] https://crrev.com/9bc80e246d791df3e093bf2b98986e7fab36934a/ui/views/style/platform_style.h
[modify] https://crrev.com/9bc80e246d791df3e093bf2b98986e7fab36934a/ui/views/style/platform_style_mac.mm
[delete] https://crrev.com/6ffce94e929ce46e50e364b726e07e2b4773c82b/ui/views/vector_icons/combobox_arrow_mac_disabled.icon
[delete] https://crrev.com/6ffce94e929ce46e50e364b726e07e2b4773c82b/ui/views/vector_icons/combobox_arrow_mac_enabled.icon

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4ae3b7003dfa719948403cfceb9410028ac47835

commit 4ae3b7003dfa719948403cfceb9410028ac47835
Author: ellyjones <ellyjones@chromium.org>
Date: Mon Apr 24 13:41:02 2017

ui: remove dead NativeThemeMac code

The code for drawing Shiny Modern style buttons is long-dead.

BUG= 629958 

Review-Url: https://codereview.chromium.org/2834173002
Cr-Commit-Position: refs/heads/master@{#466623}

[modify] https://crrev.com/4ae3b7003dfa719948403cfceb9410028ac47835/ui/native_theme/native_theme_mac.h
[modify] https://crrev.com/4ae3b7003dfa719948403cfceb9410028ac47835/ui/native_theme/native_theme_mac.mm

All references to Shiny Modern are gone, as is all the support code.