Remove content::NotificationObserver dependency from most Prefs code.

base/files/important_file_writer.h

Index: base/files/important_file_writer.h
diff --git a/base/files/important_file_writer.h b/base/files/important_file_writer.h
index f0981aa99922ed6d6dfb884883f6f7aa85677800..ae814d5e1b36fc99e703e05405a94363fc13984e 100644
--- a/base/files/important_file_writer.h
+++ b/base/files/important_file_writer.h
@@ -40,7 +40,7 @@ class BASE_EXPORT ImportantFileWriter : public NonThreadSafe {
  public:
   // Used by ScheduleSave to lazily provide the data to be saved. Allows us
   // to also batch data serializations.
-  class DataSerializer {
+  class BASE_EXPORT DataSerializer {
    public:
     // Should put serialized string in |data| and return true on successful
     // serialization. Will be called on the same thread on which