Cross-reference JS features guide from style guide

The Web style guide's JS section should link to the ES supported
features doc, and vice versa, so that both are discoverable.

Also gives the ES Support page a proper title:
"ECMAScript Features in Chromium"

Bug: None
Change-Id: I4af3471a4eba1f7377826a35c08cbb8841db411d
Reviewed-on: https://chromium-review.googlesource.com/c/1450586
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628553}

styleguide/web/es.md

@@ -83,11 +83,10 @@ document.addEventListener('DOMContentLoaded', function(event) {
 });
 </script>
 
-[TOC]
-
-> **TBD:** Do we need to differentiate per-project?
+# ECMAScript Features in Chromium
 
-> **TBD:** Cross-platform build support? As in: transpilers?
+This doc extends the [style guide](web.md#JavaScript) by specifying which new
+features of ES2015 and beyond are allowed in Chromium.
 
 You can propose changing the status of a feature by sending an email to
 chromium-dev@chromium.org. Include a short blurb on what the feature is and why
@@ -95,6 +94,8 @@ you think it should or should not be allowed, along with links to any relevant
 previous discussion. If the list arrives at some consensus, send a codereview
 to change this file accordingly, linking to your discussion thread.
 
+[TOC]
+
 # ES2015 Support In Chromium
 
 This is a list of [ECMAScript 6 a.k.a.
@@ -1134,4 +1135,4 @@ console.log(Object.entries(object2)[1]);
 
 **Discussion Notes / Link to Thread:**
 
----
\ No newline at end of file
+---

styleguide/web/web.md

@@ -302,7 +302,8 @@ For properties that don't have an RTL-friendly alternatives, use
 ### Style
 
 See the [Google JavaScript Style
-Guide](https://google.github.io/styleguide/jsguide.html).
+Guide](https://google.github.io/styleguide/jsguide.html) as well as
+[ECMAScript Features in Chromium](es.md).
 
 * Use `$('element-id')` instead of `document.getElementById`