Fix HistoryCounterTest flakiness around DST changes

The test fails when changing between winter/summer time due to days
being longer/shorter than 24h. RevertTimeInDays assumes that every day
has 24h. To fix the issue the test always runs on a fixed timestamp.

Bug: 1144634
Change-Id: I55f94df979c61a3e965885eb577bc05d41612725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523032Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Christian Dullweber <dullweber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825290}

chrome/browser/browsing_data/counters/history_counter_browsertest.cc

@@ -51,6 +51,8 @@ class HistoryCounterTest : public InProcessBrowserTest {
     return time_;
   }
 
+  void SetTime(base::Time time) { time_ = time; }
+
   void RevertTimeInDays(int days) {
     time_ -= base::TimeDelta::FromDays(days);
   }
@@ -126,6 +128,11 @@ class HistoryCounterTest : public InProcessBrowserTest {
 // Tests that the counter considers duplicate visits from the same day
 // to be a single item.
 IN_PROC_BROWSER_TEST_F(HistoryCounterTest, DuplicateVisits) {
+  // Start at a fixed day to avoid flakiness due to timezone changes.
+  base::Time time;
+  ASSERT_TRUE(base::Time::FromUTCString("1 Jul 2020 10:00 GMT", &time));
+  SetTime(time);
+
   AddVisit("https://www.google.com");   // 1 item
   AddVisit("https://www.google.com");
   AddVisit("https://www.chrome.com");   // 2 items