Support notification grouping

[amehfooz]

Android has supported notification groups for a while using a group_key field to differentiate between notification groups originating from a single app.

ChromeOS supports basic grouping for Web notifications based on the origin. However, this leads to sub-optimal outcomes like chat and email notifications from Gmail.com being grouped together.

I suggest adding an optional DOMString group_key attribute to Notification. This addition would enable more nuanced differentiation between notifications coming from a single website

What does everyone think?