DevTools: Follow Lighthouse's a11y suggestions

Running Lighthouse on DevTools lead to these changes:
 * Label our <select> toolbar items
 * Don't use tabIndex=1 for our canvas elements
 * Don't use li items in the network panel
 * add lang=en to our html tags

Change-Id: I9fef3d731dbbcbb56c3f73d81fd5669d65fa9827
Reviewed-on: https://chromium-review.googlesource.com/1093399
Reviewed-by: Dmitry Gozman <[email protected]>
Commit-Queue: Joel Einbinder <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#566195}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 237b755cd007b58b585ad34137353e11e2bc3ccd
diff --git a/front_end/node_app.html b/front_end/node_app.html
index 6b9ddcd..71c675c 100644
--- a/front_end/node_app.html
+++ b/front_end/node_app.html
@@ -4,7 +4,7 @@
  * found in the LICENSE file.
 -->
 <!doctype html>
-<html>
+<html lang="en">
 <head>
     <meta http-equiv="content-type" content="text/html; charset=utf-8">
     <meta http-equiv="Content-Security-Policy" content="object-src 'none'; script-src 'self' 'unsafe-eval' 'unsafe-inline' https://chrome-devtools-frontend.appspot.com">