MD Settings: Debounce navigation event in side nav.

Side nav was causing 2 navigation events because |setSelectedUrl_| sets
the selected URL on both selectors. Debounce will make sure only one of
these events is handled. Timeout is not necessary.

R=dschuyler@gmail.com
BUG=696458
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2733963002
Cr-Commit-Position: refs/heads/master@{#457475}

chrome/browser/resources/settings/settings_menu/settings_menu.html

@@ -133,8 +133,7 @@
         </iron-icon></paper-button>
       <iron-collapse id="advancedSubmenu" opened="[[advancedOpened]]"
           hidden="[[!pageVisibility.advancedSettings]]">
-        <iron-selector id="subMenu" selectable="a" attr-for-selected="href"
-            on-iron-activate="onSelectorActivate_">
+        <iron-selector id="subMenu" selectable="a" attr-for-selected="href">
 <if expr="chromeos">
           <a href="/dateTime">
             <iron-icon icon="settings:access-time"></iron-icon>