Support FormData on WorkerGlobalScope.

LayoutTests/fast/workers/worker-formdata-expected.txt

Index: LayoutTests/fast/workers/worker-formdata-expected.txt
diff --git a/LayoutTests/fast/workers/worker-formdata-expected.txt b/LayoutTests/fast/workers/worker-formdata-expected.txt
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+[Worker] Test FormData interface object
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+Starting worker: ./resources/worker-formdata.js
+PASS [Worker] FormData is defined.
+PASS [Worker] FormData.length is 0
+PASS [Worker] formData is non-null.
+PASS [Worker] 'append' in formData is true
+PASS [Worker] formData.append('key', 'value'); did not throw exception.
+PASS [Worker] blob is non-null.
+PASS [Worker] formData.append('key', blob); did not throw exception.
+PASS [Worker] formData.append('key', blob, 'filename'); did not throw exception.
+PASS [Worker] postMessage(formData) threw exception DataCloneError: Failed to execute 'postMessage' on 'WorkerGlobalScope': An object could not be cloned..
+PASS successfullyParsed is true
+
+TEST COMPLETE
+