fetch: Drop Headers from HeadersInit typedef declaration.

third_party/WebKit/LayoutTests/external/wpt/fetch/api/headers/headers-basic-expected.txt

-This is a testharness.js-based test.
-PASS Create headers from no parameter 
-PASS Create headers from undefined parameter 
-PASS Create headers from empty object 
-PASS Create headers with null should throw 
-PASS Create headers with 1 should throw 
-PASS Create headers with sequence 
-PASS Create headers with record 
-PASS Create headers with existing headers 
-FAIL Create headers with existing headers with custom iterator assert_equals: expected (string) "test" but got (object) null
-PASS Check append method 
-PASS Check set method 
-PASS Check has method 
-PASS Check delete method 
-PASS Check get method 
-PASS Check keys method 
-PASS Check values method 
-PASS Check entries method 
-PASS Check Symbol.iterator method 
-PASS Check forEach method 
-Harness: the test ran to completion.
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