Chrome OS: Hide ripple effect on back button when it is disabled.

This CL hides ripple effect on "Backspace" button when it is disabled.

Bug: 898830
Change-Id: Ie77e0f0846a707be4d288c1762d4775403ad32d5
Reviewed-on: https://chromium-review.googlesource.com/c/1312309Reviewed-by: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604735}

ui/webui/resources/cr_components/chromeos/quick_unlock/pin_keyboard.html

@@ -286,7 +286,8 @@
                 title="[[i18n('pinKeyboardDeleteAccessibleName')]]"
                 noink>
             </paper-icon-button>
-            <paper-ripple class="circle" center></paper-ripple>
+            <paper-ripple class="circle" center hidden="[[!hasInput_(value)]]">
+            </paper-ripple>
           </div>
         </div>
       </div>