Reland "Move notification_helper/ to chrome/notification_helper/"

This is a reland of 850001f395fe619b3add0fdb80a1cfbd7d32df01, which caused
a pre-submit deadlock for chrome/browser/notifications/OWNERS.

This is now safe to land as the following CL has been landed:
https://chromium-review.googlesource.com/c/chromium/src/+/1024633

Original change's description:
> Move notification_helper/ to chrome/notification_helper/
>
> ... plus all the necessary code migration. A follow-up CL will be used
> to update relevant comments.
>
> Bug: 734095
> Change-Id: I8516c750518e10a366bdc1496e75a3c2f4f0898b
> Reviewed-on: https://chromium-review.googlesource.com/1022957
> Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> Commit-Queue: Xi Cheng <chengx@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#552762}

Bug: 734095
Change-Id: Id4e0818d7e3ea2b0ea7633e883bc5cea42705167
Reviewed-on: https://chromium-review.googlesource.com/1024744
Commit-Queue: Xi Cheng <chengx@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Xi Cheng <chengx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552971}
25 files changed