Implement Notification.data for persistent notifications.
This patch implements the Notification.data property for persistent
notifications, where the data is stored in the Notification Database.
The data associated with non-persistent notifications will never leave
Blink, so no changes are required in Chromium to support this.
As an implementation-defined limit, the size of the data will be
limited to a maximum of 1024^2 characters, with a UMA histogram tracking
the size of what developers are trying to store. The limit can then be
adjusted in future releases to match this more accurately, or as an
argument to introduce such a limit in the specification.
Further layout tests are being introduced in the following CL:
https://codereview.chromium.org/
1052693002
BUG=442129
Review URL: https://codereview.chromium.org/
1052683002
Cr-Commit-Position: refs/heads/master@{#325295}