Added clone support in Request and Response and tests.

Added tests for bodyUsed in Response as well.

See github discussion here for clone behavior:
https://github.com/slightlyoff/ServiceWorker/issues/372#issuecomment-52751957

BUG=414486

Review URL: https://codereview.chromium.org/545983004

git-svn-id: svn://svn.chromium.org/blink/trunk@182345 bbb929c8-8fbe-4397-9dbb-9b2b20218538