Add an extension bubble explaining which extensions are in dev mode.

chrome/browser/extensions/extension_prefs.h

Index: chrome/browser/extensions/extension_prefs.h
diff --git a/chrome/browser/extensions/extension_prefs.h b/chrome/browser/extensions/extension_prefs.h
index f7130a59fdc44f84792da61b3c372c0e5796641d..bd9b83622abb8ad8766a397eccab17e712120674 100644
--- a/chrome/browser/extensions/extension_prefs.h
+++ b/chrome/browser/extensions/extension_prefs.h
@@ -302,6 +302,11 @@ class ExtensionPrefs : public ExtensionScopedPrefs,
   bool HasWipeoutBeenAcknowledged(const std::string& extension_id);
   void SetWipeoutAcknowledged(const std::string& extension_id, bool value);
 
+  // Whether the user has been notified about extension with |extension_id|
+  // only running because of dev mode being on.
+  bool HasDevModeBeenAcknowledged(const std::string& extension_id);
+  void SetDevModeAcknowledged(const std::string& extension_id, bool value);

[not at google - send to devlin, 2013/12/04 17:25:13]

why are we storing this in preferences? Isn't the point of this that we don't
store such things in preferences, and we show it on every startup?

[Finnur, 2013/12/05 23:55:56]

Indeed. Done.

+
   // 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