WebUI: cr-slider, make knob not focusable when disabled

Bug: 906009
Change-Id: Ic3ad0336522415492c043bc125ced0fa5796f169
Reviewed-on: https://chromium-review.googlesource.com/c/1341059Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609061}

ui/webui/resources/cr_elements/cr_slider/cr_slider.js

@@ -56,6 +56,7 @@ cr_slider.SliderTick;
         type: Boolean,
         computed: 'computeDisabled_(disabled, ticks.*)',
         reflectToAttribute: true,
+        observer: 'onDisabledChanged_',
       },
 
       dragging: {
@@ -249,6 +250,12 @@ cr_slider.SliderTick;
       this.holdDown_ = false;
     },
 
+    /** @private */
+    onDisabledChanged_: function() {
+      this.$.knob.setAttribute('tabindex', this.disabled_ ? '-1' : '0');
+      this.blur();
+    },
+
     /** @private */
     onFocus_: function() {
       this.holdDown_ = true;