ObserverListThreadSafe::Notify is the 4th most called function in a
profile generated on my host. This is because all tasks posted from it
are stamped with this callsite, providing very little information.

This patch forces callers to pass in their own tracking data so that
the profile will differentiate between each observer list holder.

TBR=garykac@chromium.org,rdsmith@chromium.org,stuartmorgan@chromium.org,gbillock@chromium.org

Review URL: https://codereview.chromium.org/877993003

Cr-Commit-Position: refs/heads/master@{#315372}