mac: Fix a compile error when using libc++.

chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.h

Index: chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.h
diff --git a/chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.h b/chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.h
index 58c6c1b100717e385f76dca53c7920437070dd70..701787e059a26beb3395c776e5ec7dbc939859e4 100644
--- a/chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.h
+++ b/chrome/browser/ui/cocoa/content_settings/content_setting_bubble_cocoa.h
@@ -40,7 +40,7 @@ struct MediaMenuParts {
 
 // Comparator used by MediaMenuPartsMap to order its keys.
 struct compare_button {
-  bool operator()(NSPopUpButton *const a, NSPopUpButton *const b) {
+  bool operator()(NSPopUpButton *const a, NSPopUpButton *const b) const {
     return [a tag] < [b tag];
   }
 };