[MultidevicePage] Add focus override to multidevice feature item

Adds an override to the focus() function for the multidevice feature
item so that the feature controller is focused when the item is focused.
Helps with adding deep links to the multidevice page.

Bug: 1084154
Change-Id: I9a5c382ccf5b5d6c6f6315bd4da200949e7b2c7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2417142Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Daniel Classon <dclasson@google.com>
Cr-Commit-Position: refs/heads/master@{#808110}

chrome/browser/resources/settings/chromeos/multidevice_page/BUILD.gn

@@ -47,6 +47,7 @@ js_library("multidevice_feature_item") {
     "..:route_origin_behavior",
     "../..:router",
     "../localized_link:localized_link",
+    "//ui/webui/resources/js:assert",
     "//ui/webui/resources/js:cr",
   ]
 }
@@ -183,6 +184,7 @@ js_library("multidevice_feature_item.m") {
     "../..:router.m",
     "../localized_link:localized_link.m",
     "//third_party/polymer/v3_0/components-chromium/polymer:polymer_bundled",
+    "//ui/webui/resources/js:assert.m",
   ]
   extra_deps = [ ":multidevice_feature_item_module" ]
 }
@@ -300,7 +302,8 @@ polymer_modulizer("multidevice_feature_item") {
   html_type = "dom-module"
   migrated_imports = os_settings_migrated_imports
   namespace_rewrites = os_settings_namespace_rewrites
-  auto_imports = os_settings_auto_imports
+  auto_imports = os_settings_auto_imports +
+                 [ "ui/webui/resources/html/assert.html|assert" ]
 }
 
 polymer_modulizer("multidevice_feature_toggle") {

chrome/browser/resources/settings/chromeos/multidevice_page/multidevice_feature_item.html

@@ -2,6 +2,7 @@
 
 <link rel="import" href="chrome://resources/cr_elements/cr_icon_button/cr_icon_button.html">
 <link rel="import" href="chrome://resources/cr_elements/shared_vars_css.html">
+<link rel="import" href="chrome://resources/html/assert.html">
 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="../os_route.html">

chrome/browser/resources/settings/chromeos/multidevice_page/multidevice_feature_item.js

@@ -42,6 +42,16 @@ Polymer({
     this.addFocusConfig_(this.subpageRoute, '#subpageButton');
   },
 
+  /** @override */
+  focus() {
+    const slot = this.$$('slot[name="feature-controller"]');
+    const elems = slot.assignedElements({flatten: true});
+    assert(elems.length > 0);
+    // Elems contains any elements that override the feature controller. If none
+    // exist, contains the default toggle elem.
+    elems[0].focus();
+  },
+
   /**
    * @return {boolean}
    * @private