Fix flaky EngagementTimeUtilTest test

The test still used System.currentTimeMillis() which could very rarely
return a different value even between just a few instructions.

Change-Id: I8cc649771cec6506b28decba91bd74a1ed1a7e03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929084Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718399}

chrome/android/junit/src/org/chromium/chrome/browser/tasks/EngagementTimeUtilTest.java

@@ -19,22 +19,25 @@ import org.chromium.base.test.BaseRobolectricTestRunner;
 public final class EngagementTimeUtilTest {
     private static final long TEST_ELAPSED_MS = 5000L;
 
-    private EngagementTimeUtil mEngagementTimeUtil;
+    private MockEngagementTimeUtil mEngagementTimeUtil;
 
     @Before
     public void setUp() {
-        mEngagementTimeUtil = new EngagementTimeUtil();
+        mEngagementTimeUtil = new MockEngagementTimeUtil();
     }
 
     @Test
     public void timeSinceLastEngagement_shouldReportElapsedTimeSinceLastEngagement() {
+        long currentTimeMillis = System.currentTimeMillis();
+        mEngagementTimeUtil.setCurrentTime(currentTimeMillis);
         assertEquals(TEST_ELAPSED_MS,
-                mEngagementTimeUtil.timeSinceLastEngagement(
-                        System.currentTimeMillis() - TEST_ELAPSED_MS));
+                mEngagementTimeUtil.timeSinceLastEngagement(currentTimeMillis - TEST_ELAPSED_MS));
     }
 
     @Test
     public void timeSinceLastEngagement_shouldReportInvalidIfNegative() {
+        long currentTimeMillis = System.currentTimeMillis();
+        mEngagementTimeUtil.setCurrentTime(currentTimeMillis);
         assertEquals(-1,
                 mEngagementTimeUtil.timeSinceLastEngagement(
                         System.currentTimeMillis() + TEST_ELAPSED_MS));
@@ -43,9 +46,24 @@ public final class EngagementTimeUtilTest {
     @Test
     public void timeSinceLastEngagement_shouldReportElapsedTimeBetweenTimestamps() {
         long currentTimeMillis = System.currentTimeMillis();
+        mEngagementTimeUtil.setCurrentTime(currentTimeMillis);
         assertEquals(TEST_ELAPSED_MS,
                 mEngagementTimeUtil.timeSinceLastEngagement(
                         currentTimeMillis - (2L * TEST_ELAPSED_MS),
                         currentTimeMillis - TEST_ELAPSED_MS));
     }
+
+    private static class MockEngagementTimeUtil extends EngagementTimeUtil {
+        private long mCurrentTime = System.currentTimeMillis();
+
+        public void setCurrentTime(long currentTime) {
+            mCurrentTime = currentTime;
+        }
+
+        // EngagementTimeUtil implementation.
+        @Override
+        public long currentTime() {
+            return mCurrentTime;
+        }
+    }
 }