Hotword: Make the opt-in flow tab ordering more intuitive

Makes the primary buttons first in tab order.

BUG=466390

Review URL: https://codereview.chromium.org/1011283003

Cr-Commit-Position: refs/heads/master@{#321501}

chrome/browser/resources/hotword_audio_verification/flow.js

@@ -545,12 +545,15 @@
 
   /**
    * Displays the current step. If the current step is not the first step,
-   * also hides the previous step.
+   * also hides the previous step. Focuses the current step's first button.
    * @private
    */
   Flow.prototype.showStep_ = function() {
-    var currentStep = this.currentFlow_[this.currentStepIndex_];
-    document.getElementById(currentStep).hidden = false;
+    var currentStepId = this.currentFlow_[this.currentStepIndex_];
+    var currentStep = document.getElementById(currentStepId);
+    currentStep.hidden = false;
+
+    cr.ui.setInitialFocus(currentStep);
 
     var previousStep = null;
     if (this.currentStepIndex_ > 0)

chrome/browser/resources/hotword_audio_verification/main.html

@@ -6,6 +6,8 @@
     <link rel="stylesheet" href="chrome://resources/css/text_defaults.css">
     <link type="text/css" rel="stylesheet" href="style.css">
     <script src="chrome://resources/js/action_link.js"></script>
+    <script src="chrome://resources/js/cr.js"></script>
+    <script src="chrome://resources/js/cr/ui/node_utils.js"></script>
     <script src="chrome://resources/js/load_time_data.js"></script>
     <script src="chrome://resources/js/util.js"></script>
     <script src="chrome://resources/js/i18n_template_no_process.js"></script>