Tests grouped invalidations on the timeline.

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paint invalidations[
Exception while running: function () {
    var event = PerformanceTestRunner.findTimelineEvent(TimelineModel.TimelineModel.RecordType.Paint);
    TestRunner.addArray(
        TimelineModel.InvalidationTracker.invalidationEventsFor(event), PerformanceTestRunner.InvalidationFormatters,
        '', 'paint invalidations');

    var linkifier = new Components.Linkifier();
    var target = PerformanceTestRunner.timelineModel().targetByEvent(event);
    var contentHelper = new Timeline.TimelineDetailsContentHelper(target, linkifier, true);
    Timeline.TimelineUIUtils._generateCauses(event, target, null, contentHelper);
    var invalidationsTree = contentHelper.element.getElementsByClassName('invalidations-tree')[0];
    var invalidations = invalidationsTree.shadowRoot.textContent;
    checkStringContains(
        invalidations,
        'Inline CSS style declaration was mutated for [ DIV class=\'testElement\' ], [ DIV class=\'testElement\' ], and 2 others. (anonymous) @ timeline-grouped-invalidations.html:14');
    checkStringContains(
        invalidations,
        'Inline CSS style declaration was mutated for [ DIV class=\'testElement\' ], [ DIV class=\'testElement\' ], and 2 others. (anonymous) @ timeline-grouped-invalidations.html:15');
    TestRunner.completeTest();
  }
TypeError: Cannot read property 'length' of null
    at Object.TestRunner.addArray (file:///b/c/b/mac_layout/src/out/Release/resources/inspector/integration_test_runner.js:6003:232)
    at eval (evaluateInWebInspector1.js:65:16)
    at result (file:///b/c/b/mac_layout/src/out/Release/resources/inspector/integration_test_runner.js:5943:42)
    at Timeline.TimelinePanel.TestRunner.addSniffer (file:///b/c/b/mac_layout/src/out/Release/resources/inspector/performance_test_runner/performance_test_runner_module.js:5:1190)
    at Timeline.TimelinePanel.result (file:///b/c/b/mac_layout/src/out/Release/resources/inspector/integration_test_runner.js:5943:42)
    at Timeline.TimelinePanel.receiver.(anonymous function) (file:///b/c/b/mac_layout/src/out/Release/resources/inspector/integration_test_runner.js:5937:58)
    at Timeline.TimelineController._finalizeTrace (file:///b/c/b/mac_layout/src/out/Release/resources/inspector/timeline/timeline_module.js:183:99)
    at setTimeout (file:///b/c/b/mac_layout/src/out/Release/resources/inspector/timeline/timeline_module.js:182:76)

