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This CL implements the following classes:
 - SignedExchangeHandler
 - WebPackageLoader
 - WebPackageRequestHandler

But Currenly SignedExchangeHandler doesn't implement any CBOR parsing logic nor
verifying logic. It just behaves as if the passed body is a signed HTTP exchange
which contains a request to "https://example.com/test.html" and a response with
a payload which is equal to the original body.

The added LayoutTest "origin-signed-response.html" must pass only when
SignedHTTPExchange feature is enabled.

Bug: 803774

Change-Id: I53d397d7cdfe79d48d6891db65ab1a1e12100b92
Reviewed-on: https://chromium-review.googlesource.com/869872
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531921}