Fix forced-colors-mode-15.html flaky test

forced-colors-mode-15.html was marked as flaky on Mac.
forced-colors-mode-14.html is a similar test for active selection
rather than inactive and is not flaky. Thus, I believe the
flakiness is likely around "testRunner.setWindowFocus(false);".

Similar to CL:1146523, delay calling setWindowFocus to avoid a race with
the initial focus event.

Bug: 1167210
Change-Id: I8e6ae87d8cd9b75ea01d2e9ff012c0ebcb9e32f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2641321
Commit-Queue: Alison Maher <almaher@microsoft.com>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846378}

third_party/blink/web_tests/TestExpectations

@@ -5803,7 +5803,6 @@ crbug.com/1147859 external/wpt/css/css-pseudo/highlight-painting-order.html [ Fa
 crbug.com/1060837 virtual/plz-dedicated-worker/external/wpt/html/cross-origin-embedder-policy/dedicated-worker.https.html [ Timeout ]
 
 # Sheriff 2021-01-15
-crbug.com/1167210 [ Mac ] virtual/forced-high-contrast-colors/fast/css/forced-colors-mode/forced-colors-mode-15.html [ Pass Failure ]
 crbug.com/1167222 http/tests/websocket/multiple-connections-throttled.html [ Pass Failure ]
 crbug.com/1167309 fast/canvas/canvas-createImageBitmap-drawImage.html [ Pass Failure ]
 

third_party/blink/web_tests/virtual/forced-high-contrast-colors/fast/css/forced-colors-mode/forced-colors-mode-15.html

@@ -30,13 +30,22 @@
 </body>
 
 <script>
-  var node1 = document.getElementsByClassName('select')[0].firstChild;
-  var node2 = document.getElementsByClassName('select')[1].firstChild;
-  var range = document.createRange();
-  range.setStart(node1, 0);
-  range.setEnd(node2, node2.length);
-  window.getSelection().addRange(range);
+  function runTest() {
+    var node1 = document.getElementsByClassName('select')[0].firstChild;
+    var node2 = document.getElementsByClassName('select')[1].firstChild;
+    var range = document.createRange();
+    range.setStart(node1, 0);
+    range.setEnd(node2, node2.length);
+    window.getSelection().addRange(range);
+    if (window.testRunner) {
+      testRunner.setWindowFocus(false);
+      setTimeout(function() { testRunner.notifyDone(); }, 0);
+    }
+  }
 
-  if (window.testRunner)
-    testRunner.setWindowFocus(false);
+  if (window.testRunner) {
+    testRunner.waitUntilDone();
+    // Add some delay to avoid the initial focus event.
+    setTimeout(runTest, 100);
+  }
 </script>