Remove content::NotificationObserver dependency from most Prefs code.

base/prefs/persistent_pref_store.h

Index: base/prefs/persistent_pref_store.h
diff --git a/base/prefs/persistent_pref_store.h b/base/prefs/persistent_pref_store.h
index 9dd0ff967bbeda21b45330b4fc8e3794977ac05b..7f5b97ccc99a0022be04ce8b1549297878201671 100644
--- a/base/prefs/persistent_pref_store.h
+++ b/base/prefs/persistent_pref_store.h
@@ -7,12 +7,13 @@
 
 #include <string>
 
+#include "base/prefs/base_prefs_export.h"
 #include "base/prefs/pref_store.h"
 
 // This interface is complementary to the PrefStore interface, declaring
 // additional functionality that adds support for setting values and persisting
 // the data to some backing store.
-class PersistentPrefStore : public PrefStore {
+class BASE_PREFS_EXPORT PersistentPrefStore : public PrefStore {
  public:
   // Unique integer code for each type of error so we can report them
   // distinctly in a histogram.