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Previously, we instantiated an NTPUserDataLogger per web contents. And
it is possible to navigate to multiple NTPs in a single web contents.
This had a few drawbacks:
* Logging should be scoped to a single NTP. Navigations from and to NTPs
  had to be tracked to reset certain logging state.
* It was harder to reason about the life time of a logger and mismatched
  life times might have caused crashes in the WebUI NTP.
* There is a race condition on the WebUI when navigating quickly from
  and to an NTP that can cause incorrect data logging and hits a DCHECK.
  See crbug/1084360 for details.

Bug: 1144317
Fixed: 1084360
Change-Id: I02f683034aa17bc12efe9b58cae3c15c4d0ee038
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586281
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839209}