Remove content::NotificationObserver dependency from most Prefs code.

base/prefs/pref_store.h

Index: base/prefs/pref_store.h
diff --git a/base/prefs/pref_store.h b/base/prefs/pref_store.h
index 104bd63ddf67d7648bba960701dbc028f72b48d0..264c1d4f1dc60699504ca20ce8fccb0f8ebf1429 100644
--- a/base/prefs/pref_store.h
+++ b/base/prefs/pref_store.h
@@ -25,7 +25,7 @@ class Value;
 class BASE_PREFS_EXPORT PrefStore : public base::RefCounted<PrefStore> {
  public:
   // Observer interface for monitoring PrefStore.
-  class Observer {
+  class BASE_PREFS_EXPORT Observer {
    public:
     // Called when the value for the given |key| in the store changes.
     virtual void OnPrefValueChanged(const std::string& key) = 0;