[ServiceWorkerCacheStorage] Blink changes to remove create/get and add open function

As per recent spec changes, remove create and get and replace it with open.

CLs in this three-legged-patch:

1. Add open to browser - https://codereview.chromium.org/664433003/
*2. Add open (remove get/create) to Blink - https://codereview.chromium.org/638023003
3. Remove get/create from browser - https://codereview.chromium.org/663503002/

BUG=423942

Review URL: https://codereview.chromium.org/638023003

git-svn-id: svn://svn.chromium.org/blink/trunk@183895 bbb929c8-8fbe-4397-9dbb-9b2b20218538