Add API to get a folder's size.

chrome/common/extensions/api/file_manager_private_internal.idl

Index: chrome/common/extensions/api/file_manager_private_internal.idl
diff --git a/chrome/common/extensions/api/file_manager_private_internal.idl b/chrome/common/extensions/api/file_manager_private_internal.idl
index 39739f2f37a4e14475cd2789aba622ae2613d2ec..3f8dbea674c00bc5ca5fcfcf0d1b54705b246e38 100644
--- a/chrome/common/extensions/api/file_manager_private_internal.idl
+++ b/chrome/common/extensions/api/file_manager_private_internal.idl
@@ -30,6 +30,7 @@ namespace fileManagerPrivateInternal {
   callback StartCopyCallback = void(long copyId);
   callback ZipSelectionCallback = void(optional boolean success);
   callback ValidatePathNameLengthCallback = void(boolean result);
+  callback GetDirectorySizeCallback = void(double size);
 
   interface Functions {
     static void resolveIsolatedEntries(DOMString[] urls,
@@ -89,5 +90,7 @@ namespace fileManagerPrivateInternal {
         DOMString parentUrl,
         DOMString name,
         ValidatePathNameLengthCallback callback);
+    static void getDirectorySize(DOMString url,
+                                 GetDirectorySizeCallback callback);
   };
 };