Add an extension override bubble and warning box for proxy extensions.

extensions/browser/extension_prefs.h

Index: extensions/browser/extension_prefs.h
diff --git a/extensions/browser/extension_prefs.h b/extensions/browser/extension_prefs.h
index c6066c3f0aebac4a885e395999b1b9e8aa33509f..a9e86378ccaf06e82bf06bd07f851ee4bee8689b 100644
--- a/extensions/browser/extension_prefs.h
+++ b/extensions/browser/extension_prefs.h
@@ -331,6 +331,13 @@ class ExtensionPrefs : public ExtensionScopedPrefs, public KeyedService {
   void SetNtpOverriddenBubbleBeenAcknowledged(const std::string& extension_id,
                                               bool value);
 
+  // Whether the user has been notified about extension with |extension_id|
+  // overriding the proxy.
+  bool HasProxyOverriddenBubbleBeenAcknowledged(

[Devlin, 2014/05/20 17:26:31]

There was an effort awhile back to try and make ExtensionPrefs an abstract
storage area, rather than know about every individual thing.  Seeing all these
Has*BeenAcknowledged() and Set*Acknowledged() reminds me of why.  I think, at
some point, it'd be nicer to see the callers just use ReadPrefAsBoolean() and
UpdateExtensionPref(), since they're really not any longer and it reduces
clutter in ExtensionPrefs.  (There's also the concern of knowledge layering
violations - even though there are no illegal includes, I don't think top-level
/extensions should know about things like ProxyOverride bubbles, which are
inherently chrome-y.)

That said, that would be a larger change (since there are all the others, too),
so it shouldn't block this CL.  But, for the future. :)

[Finnur, 2014/05/21 16:30:32]

Yeah, I've been pondering something along the same lines. Maybe I'll go back and
clean this up in a separate CL.

+      const std::string& extension_id);
+  void SetProxyOverriddenBubbleBeenAcknowledged(const std::string& extension_id,
+                                                bool value);
+
   // Returns true if the extension notification code has already run for the
   // first time for this profile. Currently we use this flag to mean that any
   // extensions that would trigger notifications should get silently