Add missing FileReader argument typechecking.

LayoutTests/fast/files/workers/worker-read-file-async-expected.txt

Index: LayoutTests/fast/files/workers/worker-read-file-async-expected.txt
diff --git a/LayoutTests/fast/files/workers/worker-read-file-async-expected.txt b/LayoutTests/fast/files/workers/worker-read-file-async-expected.txt
index 1d4104bcc1daf709caec118a1435e22bf6122013..42290e0ddd4b3a63f77bbc770e701daf09d20d72 100644
--- a/LayoutTests/fast/files/workers/worker-read-file-async-expected.txt
+++ b/LayoutTests/fast/files/workers/worker-read-file-async-expected.txt
@@ -174,10 +174,10 @@ result: data:text/plain;base64,SGVsbG8=
 Received loadend event
 Test calling multiple concurrent read methods
 readyState: 0
-Received exception, code: 11, name: InvalidStateError, message: An attempt was made to use an object that is not, or is no longer, usable.
-Received exception, code: 11, name: InvalidStateError, message: An attempt was made to use an object that is not, or is no longer, usable.
-Received exception, code: 11, name: InvalidStateError, message: An attempt was made to use an object that is not, or is no longer, usable.
-Received exception, code: 11, name: InvalidStateError, message: An attempt was made to use an object that is not, or is no longer, usable.
+Received exception, code: 11, name: InvalidStateError, message: Failed to execute 'readAsArrayBuffer' on 'FileReader': The object is already busy reading Blobs.
+Received exception, code: 11, name: InvalidStateError, message: Failed to execute 'readAsBinaryString' on 'FileReader': The object is already busy reading Blobs.
+Received exception, code: 11, name: InvalidStateError, message: Failed to execute 'readAsText' on 'FileReader': The object is already busy reading Blobs.
+Received exception, code: 11, name: InvalidStateError, message: Failed to execute 'readAsDataURL' on 'FileReader': The object is already busy reading Blobs.
 Received loadstart event
 readyState: 1
 Received load event