[DevicePage] Make KeyboardTest works well with actual setTimeout 0

Currently SetTimeout(..., 0) is clamped up to 1ms while which is
not supposed to be in the spec. The Speedometer2 performance can boost
~1% on Windows without it based on Pinpoint perf. This CL is intended
to fix the DevicePage test that become flaky with

https://chromium-review.googlesource.com/c/chromium/src/+/2578703

Flaky test: OSSettingsDevicePageTest.KeyboardTest

Fixing it by invoking additional 'flushTasks()' after
'MockInteractions.pressAndReleaseKeyOn' to make sure
the 'onKeyUp_()' event in
https://source.chromium.org/chromium/chromium/src/+/master:ui/webui/resources/cr_elements/cr_slider/cr_slider.js;l=319
be triggered in time.

Bug: 402694
Change-Id: I2bb9c2a3a52861d2d2f3aaf0eff1a5267bc97671
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2613715Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Wanming Lin <wanming.lin@intel.com>
Cr-Commit-Position: refs/heads/master@{#841920}

chrome/test/data/webui/settings/chromeos/device_page_tests.js

@@ -1048,6 +1048,7 @@ cr.define('device_page_tests', function() {
         MockInteractions.pressAndReleaseKeyOn(
             keyboardPage.$$('#repeatRateSlider').$$('cr-slider'), 39, [],
             'ArrowRight');
+        await test_util.flushTasks();
         expectEquals(1000, get('xkb_auto_repeat_delay_r2'));
         expectEquals(300, get('xkb_auto_repeat_interval_r2'));