+Devlin

I'm able to reproduce this too, on both Chrome OS & Windows. I also don't see the prompt if I install/uninstall an extension, and then re-install the extension again.

Not sure if this is by design. @Devlin, do we have any limits on the number of times the bubble is shown?

Currently, we'll only show the bubble once per profile per session.  This was intentional, but can be reconsidered.  Do we want to do something different?

What was the rationale for only showing the bubble once per profile session? 

Is it because it's uncommon that the profile would add two NTP override extensions within the same session?

Once per session was because we didn't want to prompt the user about the same extension if they happened to open two tabs in the same session.  We could also limit based on extension, but it was just suitably unlikely that any user would install two extensions, and not clearly better to show the prompt and risk being noisy.

Makes sense. I could see limiting based on extension (e.g. it would only show once per profile per extension per session) but to your point, given the unlikeliness that the user would install two extensions, I think we can treat as a low priority.

If we're certain that it's desired behavior to limit based on <profile, extension> pair, then it's pretty trivial to do that.  Are we sure we want to show the user the bubble multiple times in that case?

In my view, it's appropriate for the warning to show every time a new extension overrides the user's new tab page for the first time.

Can you think of any scenario where this would be problematic?

Nope, this should probably be fine.

catmullings@, you wanna take this one on?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/478a043aaa94cce0bebbded00aabeef45fd11d85

commit 478a043aaa94cce0bebbded00aabeef45fd11d85
Author: Catherine Mullings <catmullings@chromium.org>
Date: Wed Nov 22 07:06:06 2017

Extensions: Show NTP override prompt per ntp extension per profile

Currently, the NTP override prompt only shows once per profile for any
NTP overriding extension installed. The NTP override prompt should be
displayed each time an NTP overriding extension is installed for a
profile. This CL implements such a change.

Bug:  748803 
Change-Id: I896d9360a2a6e84f8586da481eb49be2428e9df0
Reviewed-on: https://chromium-review.googlesource.com/722454
Commit-Queue: catmullings <catmullings@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: catmullings <catmullings@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518557}
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/dev_mode_bubble_delegate.cc
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/dev_mode_bubble_delegate.h
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/extension_message_bubble_controller.cc
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/extension_message_bubble_controller.h
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/extension_message_bubble_controller_unittest.cc
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/ntp_overridden_bubble_delegate.cc
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/ntp_overridden_bubble_delegate.h
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/proxy_overridden_bubble_delegate.cc
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/proxy_overridden_bubble_delegate.h
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/settings_api_bubble_delegate.cc
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/settings_api_bubble_delegate.h
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/suspicious_extension_bubble_delegate.cc
[modify] https://crrev.com/478a043aaa94cce0bebbded00aabeef45fd11d85/chrome/browser/extensions/suspicious_extension_bubble_delegate.h

10684.0.0, 68.0.3432.0