ServiceWorker: Add a Response ctor that accepts the body as a Blob

This ctor is expected to be added to the spec:
https://github.com/slightlyoff/ServiceWorker/issues/192

Eventually we'll also have ctors for String, Stream, and
ArrayBuffer bodies.

Multi-sided patch to implement blob-type fetch event
response bodies:
#1: blink-side, THIS PATCH
#2: chromium-side: https://codereview.chromium.org/304153015/
#3: blink-side: https://codereview.chromium.org/304233017

BUG=379074

Review URL: https://codereview.chromium.org/307063002

git-svn-id: svn://svn.chromium.org/blink/trunk@175350 bbb929c8-8fbe-4397-9dbb-9b2b20218538