Expose Cache Storage API in global window/worker scope

Exposes self.caches which accesses the origin's CacheStorage instance.
Requires the --enable-experimental-web-platform-features flag, otherwise
the API is only available in Service Workers.

https://slightlyoff.github.io/ServiceWorker/spec/service_worker/#cache

BUG=439389

Review URL: https://codereview.chromium.org/1032623008

git-svn-id: svn://svn.chromium.org/blink/trunk@192778 bbb929c8-8fbe-4397-9dbb-9b2b20218538
68 files changed