Bug 1017613 - Part 1 - Fetch Body and Request. r=baku

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extra : rebase_source : 2c8b82650e3669f84ff177a32a9d9b0f845e673a
Bug 1017613 - Part 2 - Response IDL and stubs. r=baku

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extra : rebase_source : f2f55775ed2e9fc92af829d3ce6f7ca3553b97db
Bug 1017613 - Part 3 - fetch() IDL and stubs. r=baku

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extra : rebase_source : 7783234b38b5d903dd6bd6d1e25d97c2e739f78c