[ObjC ARC] Converts ios/chrome/browser/prefs:prefs to ARC.

Automatically generated ARCMigrate commit
Notable issues:None
BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2847963002
Cr-Commit-Position: refs/heads/master@{#468643}

ios/chrome/browser/prefs/BUILD.gn

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 source_set("prefs") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "ios_chrome_pref_model_associator_client.cc",
     "ios_chrome_pref_model_associator_client.h",

ios/chrome/browser/prefs/pref_observer_bridge.h

@@ -26,7 +26,7 @@ class PrefObserverBridge {
  private:
   virtual void OnPreferenceChanged(const std::string& pref_name);
 
-  id<PrefObserverDelegate> delegate_;
+  __unsafe_unretained id<PrefObserverDelegate> delegate_;
 };
 
 #endif  // IOS_CHROME_BROWSER_PREFS_PREF_OBSERVER_BRIDGE_H_

ios/chrome/browser/prefs/pref_observer_bridge.mm

@@ -7,6 +7,10 @@
 #include "base/bind.h"
 #include "components/prefs/pref_change_registrar.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 PrefObserverBridge::PrefObserverBridge(id<PrefObserverDelegate> delegate)
     : delegate_(delegate) {
 }