[MD] Implement incognito colors as a NativeTheme (for Aura).

ui/views/views_delegate.h

Index: ui/views/views_delegate.h
diff --git a/ui/views/views_delegate.h b/ui/views/views_delegate.h
index 16ecca03ce7f543c5f76dc7e994642fdaf0f883e..9297a8b71b3283f03a8d2b2619f9bd812d9655fa 100644
--- a/ui/views/views_delegate.h
+++ b/ui/views/views_delegate.h
@@ -38,6 +38,7 @@ class Rect;
 
 namespace ui {
 class ContextFactory;
+class NativeTheme;
 }
 
 namespace views {
@@ -191,6 +192,10 @@ class VIEWS_EXPORT ViewsDelegate {
   // Returns a blocking pool task runner given a TaskRunnerType.
   virtual scoped_refptr<base::TaskRunner> GetBlockingPoolTaskRunner();
 
+  // Returns a value which, if non-null, will be used as the NativeTheme for
+  // |widget| in place of the platform default native theme.
+  virtual ui::NativeTheme* GetNonDefaultNativeTheme(const Widget* widget);

[Peter Kasting, 2015/11/11 00:34:46]

Can this return the platform default native theme in cases where there's no
override?  If so, we can just name this GetNativeTheme() and maybe always use it
from Widget?

If not, maybe GetNativeThemeOverride() would be slightly better, since
"non-default" implies it's returning some sort of secondary theme so that
callers can use both, or something strange.

[Evan Stade, 2015/11/11 01:05:32]

I don't know how we'd do this because ViewsDelegate::GetInstance() can
apparently be null (also, adding implementation to the base ViewsDelegate seems
bad, and adding it to ChromeViewsDelegate is impossible because it needs access
to internal apis).
I'm fine with the Override name.

[Peter Kasting, 2015/11/11 01:25:47]

I meant for Widget to so something like:

  return views_delegate ? views_delegate->GetNativeTheme() : GetNativeTheme();

So we'd still need to null-check, it's just that we wouldn't need to also check
the return value from this.
If it's technically infeasible, OK.  I wasn't sure.

+
  protected:
   ViewsDelegate();