Gtk3: Render a GtkHeaderBar as the background of the tab strip

ui/native_theme/native_theme.h

Index: ui/native_theme/native_theme.h
diff --git a/ui/native_theme/native_theme.h b/ui/native_theme/native_theme.h
index 5f7a9d9ba5d82d55801f647f807d92fc3a029a87..d25806ca42fe844f90731e3dc18fcf0a8171a7d4 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -45,6 +45,7 @@ class NATIVE_THEME_EXPORT NativeTheme {
   // The part to be painted / sized.
   enum Part {
     kCheckbox,
+    kFrameTopArea,
     kInnerSpinButton,
     kMenuList,
     kMenuPopupBackground,
@@ -109,6 +110,11 @@ class NATIVE_THEME_EXPORT NativeTheme {
     SkColor background_color;
   };
 
+  struct FrameTopAreaExtraParams {
+    bool is_active;
+    SkColor default_background_color;  // The theme can ignore this color.

[Tom (Use chromium acct), 2017/01/11 20:07:27]

pkasting@:  I was thinking of alternative ways to do this and one thing that I
thought of was to get rid of this constant, have FrameBackground always render
the solid color, change NativeThemeBase::PaintFrameTopArea be a no-op, and have
the NativeThemeGtk3::PaintFrameTopArea override do the actual drawing.  The
disadvantage to this would be redundantly drawing the top area in
FrameBackground on Linux.  wdyt?

[Peter Kasting, 2017/01/12 18:53:53]

I think if I were going to change this, it would be to try to avoid routing
through NativeTheme at all.  It feels weird somehow that we're calling this a
"theme part".  Maybe that's just because of my Windows-centric view of the
world, where this isn't a theme part.  I was imagining
FrameBackground::PaintFrameTopArea() doing this painting directly (via ifdefs),
or directly calling code in platform-specific files that would do this, rather
than indirecting through NativeTheme.  But maybe we can't easily do the right
thing on Linux through that route.

If we need to go through NativeTheme to begin with, then I think the existing
way you've done it is OK.  I wouldn't make the change you propose.

Separately, I find the comment "The theme can ignore this color" confusing. 
NativeTheme doesn't ignore this color, and it's not really clear which "theme"
you're saying can ignore this and why.

[Tom (Use chromium acct), 2017/01/12 21:51:09]

Changed the comment, hopefully this one's better

+  };
+
   struct InnerSpinButtonExtraParams {
     bool spin_up;
     bool read_only;
@@ -207,6 +213,7 @@ class NATIVE_THEME_EXPORT NativeTheme {
     ExtraParams(const ExtraParams& other);
 
     ButtonExtraParams button;
+    FrameTopAreaExtraParams frame_top_area;
     InnerSpinButtonExtraParams inner_spin;
     MenuArrowExtraParams menu_arrow;
     MenuCheckExtraParams menu_check;