Rejigger ThemeService: move exposure of ThemeProvider interface to a

ui/base/default_theme_provider.h

Index: ui/base/default_theme_provider.h
diff --git a/ui/base/default_theme_provider.h b/ui/base/default_theme_provider.h
index 173442deaa246cd58a08a14aa6ee332b342d89c6..78ea3a3e2dc5ec7893253b273b4f234d6fd3bef5 100644
--- a/ui/base/default_theme_provider.h
+++ b/ui/base/default_theme_provider.h
@@ -24,7 +24,6 @@ class UI_BASE_EXPORT DefaultThemeProvider : public ThemeProvider {
   ~DefaultThemeProvider() override;
 
   // Overridden from ui::ThemeProvider:
-  bool UsingSystemTheme() const override;
   gfx::ImageSkia* GetImageSkiaNamed(int id) const override;
   SkColor GetColor(int id) const override;
   int GetDisplayProperty(int id) const override;
@@ -34,6 +33,7 @@ class UI_BASE_EXPORT DefaultThemeProvider : public ThemeProvider {
       const override;
 
 #if defined(OS_MACOSX)
+  bool UsingSystemTheme() const override;
   NSImage* GetNSImageNamed(int id) const override;
   NSColor* GetNSImageColorNamed(int id) const override;
   NSColor* GetNSColor(int id) const override;