Add Cache interface for ServiceWorker behind a flag

This introduces Cache interface that will provide programmable application
cache mechanism on ServiceWorker.


BUG=345640
TEST=compile (tests will be added after ServiceWorkerGlobalScope gets available in LayoutTests)

Review URL: https://codereview.chromium.org/176833003

git-svn-id: svn://svn.chromium.org/blink/trunk@168024 bbb929c8-8fbe-4397-9dbb-9b2b20218538