[Media Controls] Try minmizing volume slider layout test timeout

Change-Id: I26e660eb8146a9f4aa856ba2f972b715ca25ce3a
Reviewed-on: https://chromium-review.googlesource.com/c/1347572
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610285}

third_party/WebKit/LayoutTests/media/media-controls.js

@@ -428,9 +428,9 @@ function runAfterDoubleTapTimerFired(func) {
 }
 
 function hoverMuteButton(video, func) {
-  // 300ms slack because the test could be flaky on Mac Test build
+  // 150ms slack because the test could be flaky on Mac Test build
   // even with volume slider delay set to 0ms
-  const delayedCallback = function() { setTimeout(func, 300); };
+  const delayedCallback = function() { setTimeout(func, 150); };
   const muteBtn = muteButton(video);
   hoverOverControl(muteBtn, delayedCallback);
 }