[ServiceWorkerCache] Enable ServiceWorkerCache

This is tentative per TPM guidance and will be reverted if the intent-to-ship sinks or the feature is found too unstable.

BUG=426977

Review URL: https://codereview.chromium.org/649123003

git-svn-id: svn://svn.chromium.org/blink/trunk@184386 bbb929c8-8fbe-4397-9dbb-9b2b20218538