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This CL corrects the load time metrics for Logos on the NTP.

The old behavior:
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If the doodle cache is empty, the LogoShownTime will be recorded when
the next DoodleService#Refresh triggers the observer who requested a
Refresh.
If the requested refresh is skipped, the observer records the time when
the next notification happens due to a refresh triggered by a different
caller. This can be timely but is much more likely to be 15 min or more
in the future. This delay blurs the metric.

The wanted/new behavior:
------------------------
This CL introduces a way to know that the refresh was skipped.
The observer that records LogoShownTime uses it to prevent recording
any metrics for this case.

BUG=713166

Review-Url: https://codereview.chromium.org/2833473002
Cr-Commit-Position: refs/heads/master@{#467989}