[CrOS MultiDevice] Update setup flow buttons

Update the multi-device setup flow button bar to include the
following buttons: Back, Cancel, Forward. Set the buttons
on each page to conform to the following specs (sans padding):
https://drive.google.com/file/d/1UqHV67OrdGT0_G0L98nV5IVr26OkHu9F/view.

Screenshots with this patch applied:
start screen: https://drive.google.com/open?id=1QlWKjesaBpt0ZtNQ_vONBqu1czUWiezz
start screen (OOBE): https://drive.google.com/open?id=1kh9UNYjLaSbP-zE0U89sniZSb9Sbhk8fMw
password screen: https://drive.google.com/open?id=1olKePztZ9pqxPHupVCdrwEBZ1pyfzLA_
success screen: https://drive.google.com/open?id=1zpZfnowdpOvge8nteqsjBHuUin_I9HgW

Bug: 889574
Change-Id: I7a0834b58b6f2458e5ab6641aa5b323cb5fc32fe
Tested: Manual; ./out/Default/browser_tests --gtest_filter=*MultiDevice*
Reviewed-on: https://chromium-review.googlesource.com/c/1254760
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599737}

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

@@ -86,9 +86,9 @@
         on-setup-exited="onExitRequested_"
         forward-button-text="{{forwardButtonText_}}"
         forward-button-disabled="{{forwardButtonDisabled_}}"
-        backward-button-text="{{backwardButtonText_}}">
-      <oobe-text-button slot="backward-button">
-        <div>[[backwardButtonText_]]</div>
+        cancel-button-text="{{cancelButtonText_}}">
+      <oobe-text-button slot="cancel-button">
+        <div>[[cancelButtonText_]]</div>
       </oobe-text-button>
       <oobe-next-button id="next-button" slot="forward-button"
           disabled$="[[forwardButtonDisabled_]]">

chrome/browser/resources/chromeos/login/multidevice_setup_first_run.js

@@ -75,10 +75,10 @@ cr.define('multidevice_setup', function() {
       },
 
       /**
-       * Text to be shown on the backward navigation button.
+       * Text to be shown on the cancel button.
        * @private {string|undefined}
        */
-      backwardButtonText_: {
+      cancelButtonText_: {
         type: String,
         value: '',
       },

chrome/browser/resources/chromeos/multidevice_setup/multidevice_setup_post_oobe.html

@@ -9,6 +9,7 @@
   <template>
     <style include="multidevice-setup-shared">
       #backward-button,
+      #cancel-button,
       #forward-button {
         align-items: center;
         display: flex;
@@ -21,11 +22,16 @@
         on-forward-button-focus-requested="onForwardButtonFocusRequested_"
         forward-button-text="{{forwardButtonText_}}"
         forward-button-disabled="{{forwardButtonDisabled_}}"
+        cancel-button-text="{{cancelButtonText_}}"
         backward-button-text="{{backwardButtonText_}}">
       <paper-button id="backward-button"
           slot="backward-button" class="cancel-button">
         [[backwardButtonText_]]
       </paper-button>
+      <paper-button id="cancel-button"
+          slot="cancel-button" class="cancel-button">
+        [[cancelButtonText_]]
+      </paper-button>
       <paper-button id="forward-button"
           slot="forward-button" class="action-button"
           disabled$="[[forwardButtonDisabled_]]">

chrome/browser/resources/chromeos/multidevice_setup/multidevice_setup_post_oobe.js

@@ -30,6 +30,15 @@ Polymer({
       value: false,
     },
 
+    /**
+     * Text to be shown on the cancel navigation button.
+     * @private {string|undefined}
+     */
+    cancelButtonText_: {
+      type: String,
+      value: '',
+    },
+
     /**
      * Text to be shown on the backward navigation button.
      * @private {string|undefined}