Added limitation to notifications polyfill manifest.
Added limitation to notifications polyfill manifest.

Added a warning to the notifications polyfill about the limitation where
the user can only retrieve notifications sent from a window or
service worker if they are calling getAll from that window or service
worker.

R=raymes@chromium.org

Review URL: https://codereview.chromium.org/1586413002 .

chromium/caterpillar,chromium/caterpillar,chromium/caterpillar