Reland "[DevTools] Enable PushMessaging & Notifications in handler."

This is a reland of 972aadda46536c2d26eda8c8a6d1098ce4bf961f

Original change's description:
> [DevTools] Enable PushMessaging & Notifications in handler.
>
> Allows the Background Services handler to receive push & notification
> calls from the frontend.
>
> Bug: 927726
> Change-Id: Iaaca0205890f940d187f3b6e145e57fb1d3fdcb2
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609705
> Commit-Queue: Rayan Kanso <rayankans@chromium.org>
> Reviewed-by: Alexei Filippov <alph@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#660806}

TBR=alph@chromium.org

Bug: 927726
Change-Id: I6f4a0f43e4fc1299d5c6f7b2c7ee1c94ac837c06
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1619702
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661418}
3 files changed