Remove content::NotificationObserver dependency from most Prefs code.

chrome/browser/chromeos/drive/drive_sync_client.cc

Index: chrome/browser/chromeos/drive/drive_sync_client.cc
diff --git a/chrome/browser/chromeos/drive/drive_sync_client.cc b/chrome/browser/chromeos/drive/drive_sync_client.cc
index f5273b3e74743f92344160add1e48aff4a1bdc1b..a303cb7fd111d28514f1ceafb3dc9a589f73041c 100644
--- a/chrome/browser/chromeos/drive/drive_sync_client.cc
+++ b/chrome/browser/chromeos/drive/drive_sync_client.cc
@@ -431,9 +431,8 @@ void DriveSyncClient::OnUploadFileComplete(const std::string& resource_id,
   DoSyncLoop();
 }
 
-void DriveSyncClient::Observe(int type,
-                              const content::NotificationSource& source,
-                              const content::NotificationDetails& details) {
+void DriveSyncClient::OnPreferenceChanged(PrefServiceBase* service,
+                                          const std::string& pref_name) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
 
   // Resume the sync loop if gdata preferences are changed. Note that we