Chrome OS OOBE: make back button blue on white.

Bug: 867711,867706
Change-Id: Ibbe9f7abda41b5f6d5a234ad78705b5e1dcf484a
Reviewed-on: https://chromium-review.googlesource.com/1182948
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584712}

chrome/browser/resources/chromeos/login/oobe_buttons.html

@@ -93,7 +93,7 @@
     <link rel="stylesheet" href="oobe_text_buttons.css">
     <link rel="stylesheet" href="oobe_back_button.css">
     <paper-button id="button" on-tap="onClick_" disabled="[[disabled]]"
-        aria-label$="[[labelForAria]]" class="action-button">
+        aria-label$="[[labelForAria]]">
       <div class="flex horizontal layout start center">
         <hd-iron-icon
             icon1x="oobe-buttons-20:arrow-forward"