Reland "[DevTools] Add Notifications & Push Messaging views."

This reverts commit 7bfecf258f220ea375ffd376608d6ec71ca9d8ce.

Reason for revert: Revert my revert for breaking compilation

Original change's description:
> Revert "[DevTools] Add Notifications & Push Messaging views."
> 
> This reverts commit 793bf4990a4d8a6bffb0007e5f47f26e15791632.
> 
> Reason for revert: Guessing this is responsible for build failures on WebKit Mac10.13 (retina). See https://ci.chromium.org/p/chromium/builders/ci/WebKit%20Mac10.13%20%28retina%29/14445 for first failure.
> 
> Original change's description:
> > [DevTools] Add Notifications & Push Messaging views.
> > 
> > Add basic UI for Notifications & Push Messaging in Application >
> > Background Services.
> > One UI flag per feature is created.
> > 
> > TBR=caseq@chromium.org
> > 
> > Screenshot: crbug.com/942174#c28
> > Bug: 942174
> > Change-Id: Ifcdca9d3d78593968b0bd93d02d9dfea9419b822
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611612
> > Commit-Queue: Rayan Kanso <rayankans@chromium.org>
> > Reviewed-by: Alexei Filippov <alph@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#661384}
> 
> TBR=alph@chromium.org,caseq@chromium.org,pfeldman@chromium.org,rayankans@chromium.org
> 
> Change-Id: I3ce9bf8bf06d4df371b8c2af2130b9d9dfce79d7
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 942174
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621408
> Reviewed-by: Fergus Dall <sidereal@google.com>
> Commit-Queue: Fergus Dall <sidereal@google.com>
> Cr-Commit-Position: refs/heads/master@{#661533}

TBR=alph@chromium.org,caseq@chromium.org,pfeldman@chromium.org,rayankans@chromium.org,sidereal@google.com

Change-Id: I443ed2402908d9b43e68f5467281d7fdf2a89233
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 942174
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621468
Reviewed-by: Fergus Dall <sidereal@google.com>
Commit-Queue: Fergus Dall <sidereal@google.com>
Cr-Commit-Position: refs/heads/master@{#661544}
4 files changed