Add metrics to track type of changes to untouched autogenerated keywords

This CL stems from the need to investigate a huge drop in local
creations as noted by sync.

Metrics changes:
- Sync.SearchEngine.UntouchedAutogenerated{Added,Updated,Deleted}: A new
boolean metric which notes when untouched autogenerated keywords are
added, updated or deleted. Subtly, it will also be used to track the
count of adds, updates and deletes sent to sync.
- Sync.SearchEngine.UntouchedAutogenerated{Added,Updated,Deleted}
{IsPrepopulatedEntry,IsStartedPackEntry}: Repurposed from
Sync.SearchEngine.UntouchedAutogeneratedChanged
{IsPrepopulatedEntry,IsStartedPackEntry} to track adds, updates and
deletions separately.
- Sync.SearchEngine.NewChangesCommittedUponSyncStart: New metric to
count the number of changes committed upon MergeDataAndStartSyncing,
i.e. browser startup and initial sync.

OBSOLETE_HISTOGRAMS=Histograms Sync.SearchEngine.UntouchedAutogeneratedChanged.{IsPrepopulatedEntry,IsStartedPackEntry} are replaced by Sync.SearchEngine.UntouchedAutogenerated{Added,Updated,Deleted}.{IsPrepopulatedEntry,IsStartedPackEntry}

Change-Id: Id83b25cd388bc8372839eada6022496ac1db0647
Bug: 409731586
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6441132
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Ankush Singh <ankushkush@google.com>
Cr-Commit-Position: refs/heads/main@{#1445526}