Enable navigator.storage.estimate() in stable

Intent to Ship: https://groups.google.com/a/chromium.org/d/topic/blink-dev/frMdM1H8jJ8/discussion
Spec: https://storage.spec.whatwg.org
Status: https://www.chromestatus.com/feature/5630353511284736

BUG=603992

Review-Url: https://codereview.chromium.org/2894953004
Cr-Commit-Position: refs/heads/master@{#475032}
9 files changed