Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

extensions/browser/api/storage/settings_namespace.cc

Index: extensions/browser/api/storage/settings_namespace.cc
diff --git a/chrome/browser/extensions/api/storage/settings_namespace.cc b/extensions/browser/api/storage/settings_namespace.cc
similarity index 75%
rename from chrome/browser/extensions/api/storage/settings_namespace.cc
rename to extensions/browser/api/storage/settings_namespace.cc
index 0b7ee3c3dcf76000782efae9091de45aa09d169a..e178c7c1d844be7787e766eab1702510f6c2fa69 100644
--- a/chrome/browser/extensions/api/storage/settings_namespace.cc
+++ b/extensions/browser/api/storage/settings_namespace.cc
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "chrome/browser/extensions/api/storage/settings_namespace.h"
+#include "extensions/browser/api/storage/settings_namespace.h"
 
 #include "base/logging.h"
 
@@ -12,16 +12,20 @@ namespace settings_namespace {
 
 namespace {
 const char kLocalNamespace[] = "local";
-const char kSyncNamespace[]  = "sync";
+const char kSyncNamespace[] = "sync";
 const char kManagedNamespace[] = "managed";
 }  // namespace
 
 std::string ToString(Namespace settings_namespace) {
   switch (settings_namespace) {
-    case LOCAL:   return kLocalNamespace;
-    case SYNC:    return kSyncNamespace;
-    case MANAGED: return kManagedNamespace;
-    case INVALID: break;
+    case LOCAL:
+      return kLocalNamespace;
+    case SYNC:
+      return kSyncNamespace;
+    case MANAGED:
+      return kManagedNamespace;
+    case INVALID:
+      break;
   }
   NOTREACHED();
   return std::string();