[Fetch API] Put body members directly on Response/Request

http://fetch.spec.whatwg.org/

https://github.com/whatwg/fetch/commit/a898f9a2941350aa625aa79b24673628ac2b2a8e
Request.body and Response.body is removed from the spec.
Body interface is introduced.

interface Body {
  readonly attribute boolean bodyUsed;
  Promise<ArrayBuffer> arrayBuffer();
  Promise<Blob> blob();
  Promise<FormData> formData();
  Promise<JSON> json();
  Promise<ScalarValueString> text();
};
Request implements Body;
Response implements Body;

BUG=411743,410197,410196
TEST=http/tests/serviceworker/ http/tests/local/serviceworker/

Review URL: https://codereview.chromium.org/555443002

git-svn-id: svn://svn.chromium.org/blink/trunk@181793 bbb929c8-8fbe-4397-9dbb-9b2b20218538