Update CrSearchableDropDown to support autofocus

- Updates the CrSearchableDropDown element to support the autofocus property.

Bug: chromium:757625
Change-Id: I2df07bd7dde0a22a7b724c2bd9132c7df321a5fb
Reviewed-on: https://chromium-review.googlesource.com/1196017Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587859}

ui/webui/resources/cr_elements/cr_searchable_drop_down/cr_searchable_drop_down.html

@@ -49,7 +49,7 @@
       immediately as the user types unless the update-value-on-input flag is
       explicitly used. -->
     <cr-input label="[[label]]" on-click="onClick_" value="[[value]]"
-        on-input="onInput_" id="search">
+        on-input="onInput_" id="search" autofocus="[[autofocus]]">
     </cr-input>
     <iron-dropdown horizontal-align="left" vertical-align="top"
         vertical-offset="52">

ui/webui/resources/cr_elements/cr_searchable_drop_down/cr_searchable_drop_down.js

@@ -14,6 +14,12 @@ Polymer({
   is: 'cr-searchable-drop-down',
 
   properties: {
+    autofocus: {
+      type: Boolean,
+      value: false,
+      reflectToAttribute: true,
+    },
+
     /** @type {!Array<string>} */
     items: Array,