Extensions WebUI: replace hardcoded value causing body scroll with value from element

Bug: 850877
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Idc3c4520ab26400fbbca3490e038b1d25f4a8727
Reviewed-on: https://chromium-review.googlesource.com/1093262Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566934}

chrome/browser/resources/md_extensions/options_dialog.js

@@ -58,9 +58,9 @@ cr.define('extensions', function() {
      * @private
      */
     updateDialogSize_: function(width, height) {
-      const HEADER_HEIGHT = 64;
+      const headerHeight = this.$.body.offsetTop;
       const maxHeight = Math.min(0.9 * window.innerHeight, MAX_HEIGHT);
-      const effectiveHeight = Math.min(maxHeight, HEADER_HEIGHT + height);
+      const effectiveHeight = Math.min(maxHeight, headerHeight + height);
       const effectiveWidth = Math.max(MIN_WIDTH, width);
 
       // Get a reference to the inner native <dialog>.