Fix return type of readDirectory() to int

The return type was wrongly changed to bool in https://src.chromium.org/viewvc/blink?view=rev&revision=168897
This results in all callbacksId converted to 1 and malfunctioning DirectoryReaderSync.

BUG=347900

Review URL: https://codereview.chromium.org/196813002

git-svn-id: svn://svn.chromium.org/blink/trunk@168997 bbb929c8-8fbe-4397-9dbb-9b2b20218538