ServiceWorker: support Response.{status,statusText,headers} [blink] (2/3)

Multi-sided patch:
1/3: chromium-side, https://codereview.chromium.org/299003002/
2/3: blink-side, THIS PATCH
3/3: chromium-side, cleanup & re-enable test (n/a yet)

BUG=376733
TEST=http/tests/serviceworker/response.html
TEST=http/tests/serviceworker/headermap.html

Review URL: https://codereview.chromium.org/287363004

git-svn-id: svn://svn.chromium.org/blink/trunk@174941 bbb929c8-8fbe-4397-9dbb-9b2b20218538