net-internals: Fix same-origin policy violation.

It was modifying a style sheet from another origin to hide some UI
elements when capturing was stopped. This CL makes it add another sheet
instead.

Bug: 788061
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I734543786e80d5046b93fda1e970f9e5e40f6096
Reviewed-on: https://chromium-review.googlesource.com/794542Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519940}

chrome/browser/resources/net_internals/main.js

@@ -143,8 +143,11 @@ var MainView = (function() {
 
     stopCapturing: function() {
       g_browser.disable();
-      document.styleSheets[0].insertRule(
-          '.hide-when-not-capturing { display: none; }', 0);
+      var sheet = document.createElement('style');
+      sheet.type = 'text/css';
+      sheet.appendChild(document.createTextNode(
+          '.hide-when-not-capturing { display: none; }'));
+      document.head.appendChild(sheet);
     },
 
     initTabs_: function() {