Provision to start base::SimpleTestTickClock at initial ticks

components/memory_pressure/memory_pressure_stats_collector_unittest.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/memory_pressure/memory_pressure_stats_collector.h"
 
 #include "base/test/histogram_tester.h"
 #include "base/test/simple_test_tick_clock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 19 matching lines @@
   }
   MemoryPressureLevel last_pressure_level() const {
     return last_pressure_level_;
   }
   base::TimeTicks last_update_time() const { return last_update_time_; }
 };
 
 // Test fixture.
 class MemoryPressureStatsCollectorTest : public testing::Test {
  public:
-  MemoryPressureStatsCollectorTest() : collector_(&tick_clock_) {}
+  MemoryPressureStatsCollectorTest()
+      : tick_clock_(base::TimeTicks()), collector_(&tick_clock_) {}
 
   void Tick(int ms) {
     tick_clock_.Advance(base::TimeDelta::FromMilliseconds(ms));
   }
 
   // Validates expectations on the amount of accumulated (and unreported)
   // time (milliseconds) per pressure level.
   void ExpectAccumulated(int none_ms, int moderate_ms, int critical_ms) {
     EXPECT_EQ(base::TimeDelta::FromMilliseconds(none_ms),
               collector_.unreported_cumulative_time(0));  // None.
@@ skipping 79 matching lines @@
   EXPECT_EQ(tick_clock_.NowTicks(), collector_.last_update_time());
   ExpectAccumulated(500, 250, 0);
   ExpectReported(1, 2, 0);
   histograms_.ExpectBucketCount(
       kPressureLevelChange, UMA_MEMORY_PRESSURE_LEVEL_CHANGE_NONE_TO_MODERATE,
       1);
   histograms_.ExpectTotalCount(kPressureLevelChange, 1);
 }
 
 }  // namespace memory_pressure