Fix the drift in repeating timers.

Source/platform/TimerTest.cpp

Index: Source/platform/TimerTest.cpp
diff --git a/Source/platform/TimerTest.cpp b/Source/platform/TimerTest.cpp
index 912d1706c44439ecac46b1ede1d40506f63656c4..d5bc14d49e091e42961e54942d66bd128ca84ed9 100644
--- a/Source/platform/TimerTest.cpp
+++ b/Source/platform/TimerTest.cpp
@@ -135,6 +135,14 @@ public:
         }
     }
 
+    void runPendingTasks()
+    {
+        while (!m_timerTasks.empty() && m_timerTasks.top().runTimeSecs() <= gCurrentTimeSecs) {
+            m_timerTasks.top().run();
+            m_timerTasks.pop();
+        }
+    }
+
     bool hasOneTimerTask() const
     {
         return m_timerTasks.size() == 1;
@@ -224,6 +232,11 @@ public:
         mockScheduler()->runUntilIdle();
     }
 
+    void runPendingTasks()
+    {
+        mockScheduler()->runPendingTasks();
+    }
+
     void runUntilIdleOrDeadlinePassed(double deadline)
     {
         mockScheduler()->runUntilIdleOrDeadlinePassed(deadline);
@@ -272,6 +285,11 @@ public:
         m_runTimes.push_back(monotonicallyIncreasingTime());
     }
 
+    void recordNextFireTimeTask(Timer<TimerTest>* timer)
+    {
+        m_nextFireTimes.push_back(monotonicallyIncreasingTime() + timer->nextFireInterval());
+    }
+
     void advanceTimeBy(double timeSecs)
     {
         gCurrentTimeSecs += timeSecs;
@@ -282,6 +300,11 @@ public:
         m_platform->runUntilIdle();
     }
 
+    void runPendingTasks()
+    {
+        m_platform->runPendingTasks();
+    }
+
     void runUntilIdleOrDeadlinePassed(double deadline)
     {
         m_platform->runUntilIdleOrDeadlinePassed(deadline);
@@ -300,6 +323,7 @@ public:
 protected:
     double m_startTime;
     std::vector<double> m_runTimes;
+    std::vector<double> m_nextFireTimes;
 
 private:
     OwnPtr<TimerTestPlatform> m_platform;
@@ -736,6 +760,42 @@ TEST_F(TimerTest, DidChangeAlignmentInterval)
     EXPECT_FLOAT_EQ(m_startTime, timer.lastFireTime());
 }
 
+TEST_F(TimerTest, RepeatingTimerDoesNotDrift)
+{
+    Timer<TimerTest> timer(this, &TimerTest::recordNextFireTimeTask);
+    timer.startRepeating(2.0, FROM_HERE);
+
+    ASSERT(hasOneTimerTask());
+    recordNextFireTimeTask(&timer); // Scheduled to run at m_startTime + 2.0
+
+    advanceTimeBy(2.0);
+    runPendingTasks(); // Scheduled to run at m_startTime + 4.0
+
+    advanceTimeBy(2.1);
+    runPendingTasks(); // Scheduled to run at m_startTime + 6.0
+
+    advanceTimeBy(2.9);
+    runPendingTasks(); // Scheduled to run at m_startTime + 8.0
+
+    advanceTimeBy(3.1);
+    runPendingTasks(); // Scheduled to run at m_startTime + 12.0 (skips a beat)
+
+    advanceTimeBy(4.0);
+    runPendingTasks(); // Scheduled to run at m_startTime + 16.0 (skips a beat)
+
+    advanceTimeBy(10.0); // Scheduled to run at m_startTime + 22.0 (skips a few beats)
+    runPendingTasks();
+
+    runUntilIdleOrDeadlinePassed(m_startTime + 5.5);
+    EXPECT_THAT(m_nextFireTimes, ElementsAre(
+        m_startTime + 2.0,
+        m_startTime + 4.0,
+        m_startTime + 6.0,
+        m_startTime + 8.0,
+        m_startTime + 12.0,
+        m_startTime + 16.0,
+        m_startTime + 26.0));
+}
 
 } // namespace
 } // namespace blink