[cr147] VisitRow::source is now an optional.

In unit tests we need to add it manually.

Chromium change:
https://source.chromium.org/chromium/chromium/src/+/c7f16774c7d6195557836b150197c9c86727b62e

commit c7f16774c7d6195557836b150197c9c86727b62e
Author: Mariam Ali <alimariam@google.com>
Date:   Tue Mar 3 07:07:19 2026 -0800

    [History] Make VisitRow::source optional.

    This change wraps `VisitRow::source` in `std::optional` because we don't
    join the visits table with the visit_source table in every query and
    thus we don't always fetch the VisitSource even when it is available.

    Bug: 464528977

components/history/core/browser/brave_visit_database_unittest.cc

@@ -49,6 +49,7 @@ TEST_F(BraveVisitDatabaseTest, BraveGetKnownToSyncCount) {
   for (VisitID i = 1; i <= 3; i++) {
     VisitRow original(i, Time::Now(), 23, ui::PageTransitionFromInt(0), 19,
                       false, 0);
+    original.source = SOURCE_BROWSED;
     AddVisit(&original);
     ASSERT_EQ(i, original.visit_id);  // Verifies that we added 1, 2, and 3
   }