[ServiceWorker] Refactor Request class to make better conformance with the spec.

This change introduce referrer, mode, and credentials attributes and change the class of headers attribute from HeaderMap to Headers and make all attributes readonly. 
And also this change implements the logic which is defined in the spec. 

http://fetch.spec.whatwg.org/#request-class
http://fetch.spec.whatwg.org/#requests

BUG=373120

Review URL: https://codereview.chromium.org/329853012

git-svn-id: svn://svn.chromium.org/blink/trunk@177712 bbb929c8-8fbe-4397-9dbb-9b2b20218538