Increase timer tolerance for web test

Looking at the flakiness dashboard, all of the failures are due to
timers running within 200ms of the expected time. The test is intended
to detect the difference between timers running at 100ms intervals and
at 1000ms intervals, so the greater tolerance still provides adequate
coverage.

BUG=971590
R=alexclarke@chromium.org

Change-Id: I7b9ea31ad6dce2ba3c2cbfb9f3afa0eb8ea29ff3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758662Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688509}

third_party/blink/web_tests/TestExpectations

@@ -6290,7 +6290,6 @@ crbug.com/971319 [ Mac ] media/audio-garbage-collect.html [ Pass Timeout ]
 crbug.com/971319 [ Mac ] virtual/audio-service/media/audio-garbage-collect.html [ Pass Timeout ]
 
 # Sheriff 2019-06-06
-crbug.com/971590 [ Mac ] fast/dom/timer-throttling-hidden-page.html [ Pass Failure ]
 crbug.com/971612 fast/canvas/OffscreenCanvas-MessageChannel-transfer.html [ Pass Failure ]
 
 # Sheriff 2019-06-13

third_party/blink/web_tests/fast/dom/timer-throttling-hidden-page-expected.txt

@@ -2,12 +2,12 @@ Bug 98474: Throttle DOM timers on hidden pages.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
-PASS timerIntervalWhilePageVisible is within 100 of 100
+PASS timerIntervalWhilePageVisible is within 200 of 100
 PASS firstTimerIntervalWhilePageNotVisible is >= 0
-PASS firstTimerIntervalWhilePageNotVisible <= 1200 is true
-PASS timerIntervalWhilePageNotVisible is within 100 of 1000
-PASS timerIntervalWhilePageVisible is within 100 of 100
-PASS timerIntervalWhilePageVisible is within 100 of 100
+PASS firstTimerIntervalWhilePageNotVisible <= 1300 is true
+PASS timerIntervalWhilePageNotVisible is within 200 of 1000
+PASS timerIntervalWhilePageVisible is within 200 of 100
+PASS timerIntervalWhilePageVisible is within 200 of 100
 PASS successfullyParsed is true
 
 TEST COMPLETE

third_party/blink/web_tests/fast/dom/timer-throttling-hidden-page.html

@@ -10,7 +10,9 @@
         var firstTimerWhileNotVisible = true;
         var isPageVisible = true;
         var timeoutInterval = 100;
-        var tolerance = 100;
+        // Some of our testing hardware runs very slowly, so we need a generous
+        // tolerance value.
+        var tolerance = 200;
         var timerAlignmentInterval = 1000;
 
         function testTimer()
@@ -19,7 +21,9 @@
             if (!isPageVisible) {
                 if (firstTimerWhileNotVisible) {
                     firstTimerIntervalWhilePageNotVisible = time - previousTime;
-                    var minValue = timeoutInterval - tolerance;
+                    // Even if (tolerance > timeoutInterval), we should never
+                    // get a negative elapsed time, so clamp minValue to zero.
+                    var minValue = Math.max(0, timeoutInterval - tolerance);
                     shouldBeGreaterThanOrEqual("firstTimerIntervalWhilePageNotVisible", minValue.toString());
                     var maxValue = timeoutInterval + timerAlignmentInterval + tolerance;
                     shouldBeTrue("firstTimerIntervalWhilePageNotVisible <= " + maxValue);