GitLab

The CL:
https://codereview.chromium.org/2718583002
adds code that sends an event on the readValue callback. So if subscribes
to notifications and then calls readValue, will get notified twice. This
CL fixes this issue.

BUG=697702

Review-Url: https://codereview.chromium.org/2728623004
Cr-Commit-Position: refs/heads/master@{#455652}