[AF] Fix reporting to Autofill.Wallet* diff histograms in Directory

This CL fixes how diff metrics are reported from wallet syncable
service. Unlike the USS implementation, it reported diff metrics on
every startup, leading to inconsistencies in number of samples (and also
to inconsistencies in the distribution of the values).

This CL makes sure we do not report on startup
(in MergeDataAndStartSyncing()) but only in real updates

Bug: 908905
Change-Id: I813cc9629febdecc3f10929a73345539e12eb842
Reviewed-on: https://chromium-review.googlesource.com/c/1373451
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616981}
6 files changed