function InitializeWritableStream(stream) {
		        stream._state = 'writable';
		        // The error that will be reported by new method calls once the state becomes errored. Only set when [[state]] is
		        // 'erroring' or 'errored'. May be set to an undefined value.
		        stream._storedError = undefined;
		        stream._writer = undefined;
		        // Initialize to undefined first because the constructor of the controller checks this
		        // variable to validate the caller.
		        stream._writableStreamController = undefined;
		        // This queue is placed here instead of the writer class in order to allow for passing a writer to the next data
		        // producer without waiting for the queued writes to finish.
		        stream._writeRequests = new SimpleQueue();
		        // Write requests are removed from _writeRequests when write() is called on the underlying sink. This prevents
		        // them from being erroneously rejected on error. If a write() call is in-flight, the request is stored here.
		        stream._inFlightWriteRequest = undefined;
		        // The promise that was returned from writer.close(). Stored here because it may be fulfilled after the writer
		        // has been detached.
		        stream._closeRequest = undefined;
		        // Close request is removed from _closeRequest when close() is called on the underlying sink. This prevents it
		        // from being erroneously rejected on error. If a close() call is in-flight, the request is stored here.
		        stream._inFlightCloseRequest = undefined;
		        // The promise that was returned from writer.abort(). This may also be fulfilled after the writer has detached.
		        stream._pendingAbortRequest = undefined;
		        // The backpressure signal set by the controller.
		        stream._backpressure = false;
		    }
Performs the operation "extract a `Content-Type` value from |object|" as
specified in the specification:
https://fetch.spec.whatwg.org/#concept-bodyinit-extract

This function assumes that instance.body is present.

@param {any} body Any options.body input
@returns {string | null}