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The existing implementation for UKM.Sources.SerializedCount ends
up counting some sources that don't get serialized.

This change fixes that, versions the histogram to reflect the change,
and adds SourceIdType-specific breakouts for this histogram.

The SourceIdType-specific breakouts are useful because navigation id
sources are a more stable representation of the number of distinct
navigations being tracked by UKM.

Change-Id: I958bc36f6a37a2e7f2e02b6ba05dee939b711298
Reviewed-on: https://chromium-review.googlesource.com/c/1291594Reviewed-by: Robert Kaplow (sloooow) <rkaplow@chromium.org>
Commit-Queue: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601575}