Use consistent method to determine if a response is chunked.
Use consistent method to determine if a response is chunked.

HttpStreamParser was using one method to determine if a response is
chunked in CalculateResponseBodySize(), and another in deciding if it
should return ERR_RESPONSE_HEADERS_MULTIPLE_CONTENT_LENGTH.  Neither
method is strictly RFC 2616 compliant.  This switches to using
HttpResponseHeaders::IsChunkEncoded() when checking for multiple
distinct content length headers, so they both use the same method.

If we ever decide to strictly adhere to the RFC, then this lets us
modify just one method.

BUG=523295

Review URL: https://codereview.chromium.org/1312883003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#345727}

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