DevTools: Try to fix flaky test heap-objects-tracking

This test registered a callback that was an async function, but the
caller doesn't await the result. This means that we re-entered the
callback and modified the global counter, which was not expected.

Update the test to de-register the callback before awaiting the
stopTracking call, even though we still have an async function
where it doesn't belong, this could at least stop the flakiness.

Bug: 824904
Change-Id: Ib32eabdefb409f082dcb1f99db5f1d527fff88fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986080
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728196}

third_party/blink/web_tests/TestExpectations

@@ -4903,7 +4903,6 @@ crbug.com/824775 [ Mac ] virtual/audio-service/media/controls/video-controls-wit
 crbug.com/824848 [ Linux ] external/wpt/html/semantics/links/following-hyperlinks/activation-behavior.window.html [ Pass Failure ]
 crbug.com/824848 [ Mac ] external/wpt/html/semantics/links/following-hyperlinks/activation-behavior.window.html [ Pass Failure ]
 crbug.com/824848 [ Win ] external/wpt/html/semantics/links/following-hyperlinks/activation-behavior.window.html [ Pass Failure ]
-crbug.com/824904 inspector-protocol/heap-profiler/heap-objects-tracking.js [ Pass Failure ]
 
 # Sheriff 2018-03-26
 crbug.com/825733 [ Win ] media/color-profile-video-seek-filter.html [ Pass Timeout Failure ]

third_party/blink/web_tests/inspector-protocol/heap-profiler/heap-objects-tracking-expected.txt

@@ -26,7 +26,7 @@ HeapProfiler.lastSeenObjectId has params.lastSeenObjectId: true
 HeapProfiler.lastSeenObjectId has timestamp: true
 A heap stats fragment did arrive before HeapProfiler.lastSeenObjectId: true
 
-Number of heapStatsUpdate events >= numbrt of lastSeenObjectId events: true
+Number of heapStatsUpdate events >= number of lastSeenObjectId events: true
 At least 2 lastSeenObjectId arrived: true
 SUCCESS: tracking stopped
 

third_party/blink/web_tests/inspector-protocol/heap-profiler/heap-objects-tracking.js

 (async function(testRunner) {
-  var {page, session, dp} = await testRunner.startBlank('Test that heap tracking actually reports data fragments.');
+  const {page, session, dp} = await testRunner.startBlank('Test that heap tracking actually reports data fragments.');
 
   await session.evaluate(`
-    var junkArray = new Array(1000);
+    const junkArray = new Array(1000);
     function junkGenerator() {
-      for (var i = 0; i < junkArray.length; ++i)
+      for (let i = 0; i < junkArray.length; ++i)
         junkArray[i] = '42 ' + i;
       window.junkArray = junkArray;
     }
     setInterval(junkGenerator, 0)
   `);
 
-  var lastSeenObjectIdEventCount = 0;
-  var heapStatsUpdateEventCount = 0;
-  var fragments = [];
+  let lastSeenObjectIdEventCount = 0;
+  let heapStatsUpdateEventCount = 0;
+  let fragments = [];
 
-  dp.HeapProfiler.onLastSeenObjectId(async messageObject => {
+  const objectIdCallback = async messageObject => {
     ++lastSeenObjectIdEventCount;
     if (lastSeenObjectIdEventCount <= 2) {
-      var params = messageObject['params'];
+      const params = messageObject['params'];
       testRunner.log('HeapProfiler.lastSeenObjectId has params: ' + !!params);
       testRunner.log('HeapProfiler.lastSeenObjectId has params.lastSeenObjectId: ' + !!params.lastSeenObjectId);
       testRunner.log('HeapProfiler.lastSeenObjectId has timestamp: ' + !!params.timestamp);
@@ -26,21 +26,28 @@
       testRunner.log('');
     }
     if (lastSeenObjectIdEventCount === 2) {
-      // Wait for two updates and then stop tracing.
+      // Wait for two updates and then stop tracing. Turn off these callbacks.
+      // This callback is not awaited by the caller, meaning we can re-enter
+      // this callback while awaiting the below call. This would cause the count
+      // to get incremented again. We avoid this by de-registering the callback.
+      dp.HeapProfiler.offLastSeenObjectId(objectIdCallback);
       await dp.HeapProfiler.stopTrackingHeapObjects();
-      testRunner.log('Number of heapStatsUpdate events >= numbrt of lastSeenObjectId events: ' + (heapStatsUpdateEventCount >= lastSeenObjectIdEventCount));
+      testRunner.log('Number of heapStatsUpdate events >= number of lastSeenObjectId events: ' + (heapStatsUpdateEventCount >= lastSeenObjectIdEventCount));
       testRunner.log('At least 2 lastSeenObjectId arrived: ' + (lastSeenObjectIdEventCount >= 2));
       testRunner.log('SUCCESS: tracking stopped');
       testRunner.completeTest();
     }
-  });
+  };
+
+  dp.HeapProfiler.onLastSeenObjectId(objectIdCallback);
 
   dp.HeapProfiler.onHeapStatsUpdate(messageObject => {
     ++heapStatsUpdateEventCount;
-    var params = messageObject['params'];
-    if (heapStatsUpdateEventCount <= 2)
+    const params = messageObject['params'];
+    if (heapStatsUpdateEventCount <= 2) {
       testRunner.log('HeapProfiler.heapStatsUpdate has params: ' + !!params);
-    var statsUpdate = params.statsUpdate;
+    }
+    const statsUpdate = params.statsUpdate;
     if (heapStatsUpdateEventCount <= 2) {
       testRunner.log('HeapProfiler.heapStatsUpdate has statsUpdate: ' + !!statsUpdate);
       testRunner.log('statsUpdate length is not zero: ' + !!statsUpdate.length);