offline_metrics_collector_impl_unittest was broke due to DST

Using Time::Now() resulted in a test breakage due to daylight savings time.
Specifically, given Time t, (t+TimeDelta::FromDays(1)).LocalMidnight() results
in the same day due to DST.

To fix, I changed to use a fixed date.

Change-Id: I6278c3dce4654910589649b8f724b240ae983195
Reviewed-on: https://chromium-review.googlesource.com/c/1314614Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Commit-Queue: Dan H <harringtond@google.com>
Cr-Commit-Position: refs/heads/master@{#605044}

chrome/browser/offline_pages/prefetch/offline_metrics_collector_impl_unittest.cc

@@ -27,7 +27,9 @@ class OfflineMetricsCollectorTest : public testing::Test {
 
   // testing::Test:
   void SetUp() override {
-    test_clock().SetNow(base::Time::Now().LocalMidnight());
+    base::Time epoch;
+    ASSERT_TRUE(base::Time::FromUTCString("1 Jan 1994 GMT", &epoch));
+    test_clock().SetNow(epoch.LocalMidnight());
     OfflineMetricsCollectorImpl::RegisterPrefs(pref_service_.registry());
     Reload();
   }