Add accessibility aria labels for display zoom slider

This patch adds the required aria labels to display zoom slider.
The aria labels added are:
  - aria-valuemin
  - aria-valuemax
  - aria-valuenow
  - aria-valuetext

Bug: 791382, 790723
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ibb1c05d160027cefb91ca270beb5b5c54e3e669b
Component: Settings, display settings, display size slider
Reviewed-on: https://chromium-review.googlesource.com/1015778Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Malay Keshav <malaykeshav@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551527}

chrome/browser/resources/settings/device_page/display_size_slider.html

@@ -203,7 +203,7 @@
           </div>
         </div>
       </div>
-      <div class="bar-container">
+      <div class="bar-container" aria-hidden="true">
         <paper-progress id="sliderBar" disabled$="[[disabled]]"
             on-down="onBarDown_" on-up="onBarUp_" on-track="knobTrack_"
             value="[[index]]" min="[[min]]" max="[[max]]">

chrome/browser/resources/settings/device_page/display_size_slider.js

@@ -116,8 +116,12 @@ Polymer({
    */
   clampAndSetIndex_: function(newIndex) {
     newIndex = this.clampToRange_(newIndex, this.min, this.max);
-    if (newIndex != this.index)
+    if (newIndex != this.index) {
       this._setIndex(newIndex);
+      this.setAttribute('aria-valuenow', this.ticks[this.index].value);
+      this.setAttribute(
+          'aria-valuetext', this.getLabelForIndex_(this.ticks, this.index));
+    }
   },
 
   /**
@@ -426,6 +430,9 @@ Polymer({
     for (let i = 0; i < this.ticks.length; i++)
       if (this.ticks[i].value == this.pref.value)
         this._setIndex(i);
+    this.setAttribute('aria-valuenow', this.ticks[this.index].value);
+    this.setAttribute(
+        'aria-valuetext', this.getLabelForIndex_(this.ticks, this.index));
   },
 
   /**
@@ -466,6 +473,10 @@ Polymer({
       return;
     }
     this.max = this.ticks.length - 1;
+    this.setAttribute(
+        'aria-valuemin', this.getLabelForIndex_(this.ticks, this.min));
+    this.setAttribute(
+        'aria-valuemax', this.getLabelForIndex_(this.ticks, this.max));
     this.updateIndex_();
   },
 });