[ServiceWorkerCacheStorage] Add support for new Open method.
Create/Get have been replaced by Open which will first attempt to Get() followed by Create() if Get() failed.
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/
Note that the three CLs have been tested together and pass tests locally.
BUG=423942
Review URL: https://codereview.chromium.org/
664433003
Cr-Commit-Position: refs/heads/master@{#300047}