[cocoa] increase tab strip contrast in "Increase Contrast" mode

ui/base/theme_provider.h

Index: ui/base/theme_provider.h
diff --git a/ui/base/theme_provider.h b/ui/base/theme_provider.h
index ce98246b0513140cdc2157f419af8e6de7c264c6..fb10db6f37e6f1734954fdc49422dc6df5123297 100644
--- a/ui/base/theme_provider.h
+++ b/ui/base/theme_provider.h
@@ -101,6 +101,9 @@ class UI_BASE_EXPORT ThemeProvider {
 
   // Gets the NSGradient with the specified |id|.
   virtual NSGradient* GetNSGradient(int id) const = 0;
+
+  // Whether the "increase contrast" accessibility setting is enabled.
+  virtual bool ShouldIncreaseContrast() const = 0;
 #endif
 };