crostini: Use event.detail.value instead of querying the element

Test: Browser tests
Bug: None
Change-Id: I9b310580aa6b64649b72607019dac01862c711f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2232252
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Jason Lin <lxj@google.com>
Auto-Submit: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#776864}

chrome/browser/resources/chromeos/crostini_installer/app.html

@@ -127,7 +127,7 @@
           <div>$i18n{diskSizeHint}</div>
         </div>
         <template is="dom-if" if="[[!isLowSpaceAvailable_]]">
-          <cr-radio-group id="diskSizeRadio" selected="recommended"
+          <cr-radio-group selected="recommended"
               on-selected-changed="onDiskSizeRadioChanged_">
             <cr-radio-button name="recommended" id="recommended-size">
               $i18n{recommendedDiskSizeLabel}

chrome/browser/resources/chromeos/crostini_installer/app.js

@@ -574,10 +574,7 @@ Polymer({
 
   /** @private */
   onDiskSizeRadioChanged_(event) {
-    if (this.$$('#diskSizeRadio')) {
-      this.showDiskSlider_ =
-          (this.$$('#diskSizeRadio').selected !== 'recommended' ||
-           !!this.isLowSpaceAvailable_);
-    }
+    this.showDiskSlider_ =
+        (event.detail.value !== 'recommended' || !!this.isLowSpaceAvailable_);
   }
 });