Fetch: Update IDL to fix syntax error

[ayg]

Blocks #5665.

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Firefox (nightly channel)

Testing web-platform-tests at revision e17bffe
Using browser at version BuildID 20170426100344; SourceStamp 0f5ba06c4c5959030a05cb852656d854065e2226
Starting 10 test iterations
All results were stable

All results

1 test ran

/fetch/api/headers/headers-idl.html

Subtest	Results	Messages
	OK
Headers idl interface	FAIL	uncaught exception: Unterminated generic type record, line 2 (tokens: ', ByteString>)')\n[\n {\n "type": "other",\n "value": ","\n },\n {\n "type": "whitespace",\n "value": " "\n },\n {\n "type": "identifier",\n "value": "ByteString"\n },\n {\n "type": "other",\n "value": ">"\n },\n {\n "type": "other",\n "value": ")"\n }\n]

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Chrome (unstable channel)

Testing web-platform-tests at revision e17bffe
Using browser at version 59.0.3071.25 dev
Starting 10 test iterations
All results were stable

All results

1 test ran

/fetch/api/headers/headers-idl.html

Subtest	Results	Messages
	OK
Headers idl interface	FAIL	Uncaught Unterminated generic type record, line 2 (tokens: ', ByteString>)')\n[\n {\n "type": "other",\n "value": ","\n },\n {\n "type": "whitespace",\n "value": " "\n },\n {\n "type": "identifier",\n "value": "ByteString"\n },\n {\n "type": "other",\n "value": ">"\n },\n {\n "type": "other",\n "value": ")"\n }\n]

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