[CrOS SplitSettings] Hide browser settings banner in OS subpages.

Test:
* Manually test by clicking system tray > bluetooth section > gear
  which opens bluetooth device page directly.  The banner will not
  be present.
* Manually go to settings page and notice banner.  Then, navigate
  into a subpage and notice that the banner is not present.


Bug: 997844
Change-Id: Iefe52c0e0111f15adf6c88147a77e4e2210fd0d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773204
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691017}

chrome/browser/resources/settings/chromeos/os_settings_page/os_settings_page.html

@@ -108,7 +108,8 @@
         currentRoute_, inSearchMode, hasExpandedSection_)]]">
       <template is="dom-if"
           if="[[prefs.settings.cros.show_browser_banner.value]]">
-        <div id="browserSettingsBanner" class="settings-box banner">
+        <div id="browserSettingsBanner" hidden="[[isSubpage_]]"
+            class="settings-box banner">
           <div class="start">
             <span>$i18nRaw{browserSettingsBannerText}</span>
             <!-- Use role="presentation" because the <span> has an accessible

chrome/browser/resources/settings/chromeos/os_settings_page/os_settings_page.js

@@ -86,6 +86,12 @@ Polymer({
       computed: 'computeShowSecondaryUserBanner_(hasExpandedSection_)',
     },
 
+    /** @private {boolean} */
+    isSubpage_: {
+      type: Boolean,
+      value: false,
+    },
+
     /** @private {!settings.Route|undefined} */
     currentRoute_: Object,
   },
@@ -141,6 +147,8 @@ Polymer({
       assert(!this.hasExpandedSection_);
     }
 
+    this.isSubpage_ = newRoute.isSubpage();
+
     settings.MainPageBehavior.currentRouteChanged.call(
         this, newRoute, oldRoute);
   },