Add API to get a folder's size.

chrome/common/extensions/api/file_manager_private.idl

Index: chrome/common/extensions/api/file_manager_private.idl
diff --git a/chrome/common/extensions/api/file_manager_private.idl b/chrome/common/extensions/api/file_manager_private.idl
index fef52e9315c013a05d6fff747551f23beb21ab54..51d428a506adc4794b844faf0d5b2102ef0a4e5f 100644
--- a/chrome/common/extensions/api/file_manager_private.idl
+++ b/chrome/common/extensions/api/file_manager_private.idl
@@ -650,6 +650,9 @@ callback GetProvidingExtensionsCallback = void(ProvidingExtension[] extensions);
 // |actions| List of actions.
 callback GetCustomActionsCallback = void(fileSystemProvider.Action[] actions);
 
+// |size| result size.
+callback GetDirectorySizeCallback = void(double size);
+
 interface Functions {
   // Logout the current user for navigating to the re-authentication screen for
   // the Google account.
@@ -969,6 +972,13 @@ interface Functions {
   static void executeCustomAction([instanceof=Entry] object[] entries,
                                   DOMString actionId,
                                   SimpleCallback callback);
+
+  // Get the total size of a directory.
+  // |entry| Entry of the target directory.
+  // |callback|
+  [nocompile]
+  static void getDirectorySize([instanceof=DirectoryEntry] object entry,
+                               GetDirectorySizeCallback callback);
 };
 
 interface Events {