Bug 1651311 - Make sure TART test waits until browser window idle tasks have finished running before starting. r=sparky,perftest-reviewers

In bug 1633635, tabs were altered so that they don't animate during the first
few event loop ticks of a new window. This made it so that TART would sometimes
race the function that resumed tab animations. When TART won the race, it would
open a tab that would open without animation, and then timeout waiting for that
animation to complete.

This patch makes it so that TART waits until the first window has completed its
idle tasks, meaning that tab animations should be enabled for the window.

Differential Revision: https://phabricator.services.mozilla.com/D98399

testing/talos/talos.json

@@ -46,10 +46,7 @@
             "benchmark_zip": "jetstream.zip"
         },
         "svgr": {
-            "tests": ["tsvgx", "tsvgr_opacity", "tscrollx", "tsvg_static"]
-        },
-        "svgr_disabled_see_bug1651311": {
-            "tests": ["tart"]
+            "tests": ["tsvgx", "tsvgr_opacity", "tscrollx", "tsvg_static", "tart"]
         },
         "perf-reftest": {
             "tests": ["perf_reftest"]

testing/talos/talos/tests/tart/addon/api.js

@@ -85,7 +85,9 @@ this.tart = class extends ExtensionAPI {
         break;
 
       case "runTest":
-        new win.Tart().startTest(sendResult, command.data);
+        win.gBrowserInit.idleTasksFinishedPromise.then(() => {
+          new win.Tart().startTest(sendResult, command.data);
+        });
         break;
 
       case "setASAP":