MD Settings: Convert all usages of .bind(this) to use ES6 arrow function.

chrome/browser/resources/settings/on_startup_page/startup_url_dialog.js

Index: chrome/browser/resources/settings/on_startup_page/startup_url_dialog.js
diff --git a/chrome/browser/resources/settings/on_startup_page/startup_url_dialog.js b/chrome/browser/resources/settings/on_startup_page/startup_url_dialog.js
index 706bf18fcb542a61094cfdc987d8133eea394bd0..bed14109cdbc9a97d584b9c7c8b413a9fe749be3 100644
--- a/chrome/browser/resources/settings/on_startup_page/startup_url_dialog.js
+++ b/chrome/browser/resources/settings/on_startup_page/startup_url_dialog.js
@@ -59,18 +59,18 @@ Polymer({
         this.browserProxy_.editStartupPage(this.model.modelIndex, this.url_) :
         this.browserProxy_.addStartupPage(this.url_);
 
-    whenDone.then(function(success) {
+    whenDone.then(success => {
       if (success)
         this.$.dialog.close();
       // If the URL was invalid, there is nothing to do, just leave the dialog
       // open and let the user fix the URL or cancel.
-    }.bind(this));
+    });
   },
 
   /** @private */
   validate_: function() {
-    this.browserProxy_.validateStartupPage(this.url_).then(function(isValid) {
+    this.browserProxy_.validateStartupPage(this.url_).then(isValid => {
       this.$.actionButton.disabled = !isValid;
-    }.bind(this));
+    });
   },
 });