[cr146] GetDomainDiversity dropped V2 metrics

This is overall a simplification, as the callback doesn't produce a pair
of metrics anymore, but just the desired one.

Chromium changes:
https://chromium.googlesource.com/chromium/src/+/c63bd9318b07813ff357fc150ab02743e63b332c

commit c63bd9318b07813ff357fc150ab02743e63b332c
Author: Mark Pearson <mpearson@chromium.org>
Date:   Mon Feb 2 12:00:02 2026 -0800

    Deprecate Domain Diversity V2 metrics.

    [AI generated summary]

    This change removes the "V2" versions of the Domain Diversity metrics
    (History.DomainCount*Day_V2) which included synced visits. The
    `GetDomainDiversity` API in HistoryBackend and
    `CountUniqueDomainsVisited` in HistoryDatabase are simplified to only
    return results based on locally visited domains, as the V3 and V4
    metrics only use local counts. The corresponding unit tests and
    histogram definitions are updated accordingly.

    BUG=477024476,40896778
    OBSOLETE_HISTOGRAMS=Replaced with _V3 and _V4 versions of the histograms.

    Change-Id: Ibf52839ab6ba8afbbdadabe6cde0b3605b6c5bf0
    Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7528718
    Reviewed-by: Marc Treib <treib@chromium.org>
    Commit-Queue: Mark Pearson <mpearson@chromium.org>
    Cr-Commit-Position: refs/heads/main@{#1578302}

components/misc_metrics/page_metrics.cc

@@ -333,14 +333,13 @@ void PageMetrics::ReportFirstPageLoadTime() {
 }
 
 void PageMetrics::OnDomainDiversityResult(
-    std::pair<history::DomainDiversityResults, history::DomainDiversityResults>
-        metrics) {
-  if (metrics.first.empty() || metrics.second.empty()) {
+    history::DomainDiversityResults metrics) {
+  if (metrics.empty()) {
     return;
   }
   // The second entry in the pair counts both local, and foreign (synced)
   // visits.
-  const history::DomainMetricSet& metric_set = metrics.first.front();
+  const history::DomainMetricSet& metric_set = metrics.front();
   if (!metric_set.seven_day_metric.has_value()) {
     return;
   }

components/misc_metrics/page_metrics.h

@@ -105,9 +105,7 @@ class PageMetrics : public DefaultBrowserMonitor::Observer {
                                    uint64_t name_hash,
                                    base::HistogramBase::Sample32 sample);
 
-  void OnDomainDiversityResult(
-      std::pair<history::DomainDiversityResults,
-                history::DomainDiversityResults> result);
+  void OnDomainDiversityResult(history::DomainDiversityResults metrics);
 
   void OnBookmarkCountResult(
       std::unique_ptr<browsing_data::BrowsingDataCounter::Result> result);