[MD extensions] use .hidden rather than has/set/remove Attribute

This CL is a followup for a comment in CL 792537. There's no user facing
effect of this change. This changes the calls used to hide/unhide an the
dev tools in the toolbar.

Bug: None
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I7da1f779b48f942bfb708a4c574c0881269ca3ec
Reviewed-on: https://chromium-review.googlesource.com/798013Reviewed-by: Dan Beam (no longer on Chrome) <dbeam@chromium.org>
Commit-Queue: Dave Schuyler <dschuyler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520823}

chrome/browser/resources/md_extensions/toolbar.js

@@ -65,8 +65,8 @@ cr.define('extensions', function() {
     /** @private */
     onDevModeToggleChange_: function() {
       const drawer = this.$.devDrawer;
-      if (drawer.hasAttribute('hidden')) {
-        drawer.removeAttribute('hidden');
+      if (drawer.hidden) {
+        drawer.hidden = false;
         // Requesting the offsetTop will cause a reflow (to account for hidden).
         /** @suppress {suspiciousCode} */ drawer.offsetTop;
       }
@@ -77,11 +77,7 @@ cr.define('extensions', function() {
     onInDevModeChanged_: function() {
       // Set the initial state.
       this.expanded_ = this.inDevMode;
-      if (this.inDevMode) {
-        this.$.devDrawer.removeAttribute('hidden');
-      } else {
-        this.$.devDrawer.setAttribute('hidden', '');
-      }
+      this.$.devDrawer.hidden = !this.inDevMode;
     },
 
     /** @private */