Let the client (browser) set blobs in WebServiceWorkerResponse.

This lets ServiceWorkerCache return a response to Blink with a blob for the body.

This CL is a dependency of: https://codereview.chromium.org/622833002/

BUG=419417

Review URL: https://codereview.chromium.org/623753002

git-svn-id: svn://svn.chromium.org/blink/trunk@183500 bbb929c8-8fbe-4397-9dbb-9b2b20218538