Replace Polymer Legacy this.debounce with simple debounce utility

Change-Id: I23e4388d047062cb093144978d0c3a646bf8517f
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
index 0bb7e7e..53a2404 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
@@ -158,6 +158,7 @@
       element.reload();
       // Multiple cascading microtasks are scheduled.
       await flush();
+      await flush();
       // Reporting can be called with other parameters (ex. PluginsLoaded),
       // but only 'Diff Total Render' is important in this test.
       assert.equal(