[MD-Settings] Make the Advanced button into an actual button.

This change enables keyboard focus, as well as making the button show
as a button on screen readers.

No UI change, except that the button has a ripple now. The ripple was
missing.

BUG=626879,643309
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2305973002
Cr-Commit-Position: refs/heads/master@{#416440}

chrome/browser/resources/settings/settings_main/settings_main.html

@@ -2,6 +2,7 @@
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/promise_resolver.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
+<link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="/about_page/about_page.html">
 <link rel="import" href="/advanced_page/advanced_page.html">
 <link rel="import" href="/basic_page/basic_page.html">
@@ -13,6 +14,9 @@
   <template>
     <style>
       #advancedToggle {
+        --paper-button: {
+          text-transform: none;
+        }
         @apply(--settings-actionable);
         align-items: center;
         display: flex;
@@ -69,10 +73,10 @@
           showPages_.basic, hasExpandedSection_, previousShowPages_)]]">
         <div id="toggleSpacer"></div>
         <div id="toggleContainer">
-          <div id="advancedToggle" on-tap="toggleAdvancedPage_">
+          <paper-button id="advancedToggle" on-tap="toggleAdvancedPage_">
             <span>$i18n{advancedPageTitle}</span>
             <iron-icon icon="[[arrowState_(showPages_.advanced)]]"></iron-icon>
-          </div>
+          </paper-button>
         </div>
       </template>
       <template is="dom-if" if="[[showPages_.advanced]]">