Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

apps/shell/browser/shell_extensions_browser_client.h

Index: apps/shell/browser/shell_extensions_browser_client.h
diff --git a/apps/shell/browser/shell_extensions_browser_client.h b/apps/shell/browser/shell_extensions_browser_client.h
index d90039456516d4c25f77c76e4559ba4007ed59ca..d5c9d90dd7c9992b3c03043eb99d5d602d5bab01 100644
--- a/apps/shell/browser/shell_extensions_browser_client.h
+++ b/apps/shell/browser/shell_extensions_browser_client.h
@@ -12,6 +12,8 @@ class PrefService;
 
 namespace extensions {
 
+class ExtensionsApiClient;
+
 // An ExtensionsBrowserClient that supports a single content::BrowserContent
 // with no related incognito context.
 class ShellExtensionsBrowserClient : public ExtensionsBrowserClient {
@@ -63,6 +65,9 @@ class ShellExtensionsBrowserClient : public ExtensionsBrowserClient {
   // The single BrowserContext for app_shell. Not owned.
   content::BrowserContext* browser_context_;
 
+  // Support for extension APIs.
+  scoped_ptr<ExtensionsApiClient> api_client_;
+
   // The PrefService for |browser_context_|.
   scoped_ptr<PrefService> prefs_;