Support WebSocket in WebRequest API.

chrome/common/extensions/docs/templates/intros/webRequest.html

Index: chrome/common/extensions/docs/templates/intros/webRequest.html
diff --git a/chrome/common/extensions/docs/templates/intros/webRequest.html b/chrome/common/extensions/docs/templates/intros/webRequest.html
index a59875ad577da4da5da2c167c704978f236dcd21..0679072dd5301568f010df90a25eb73b423ebe7e 100644
--- a/chrome/common/extensions/docs/templates/intros/webRequest.html
+++ b/chrome/common/extensions/docs/templates/intros/webRequest.html
@@ -124,7 +124,9 @@ Moreover, only the following schemes are accessible:
 <code>http://</code>,
 <code>https://</code>,
 <code>ftp://</code>,
-<code>file://</code>, or
+<code>file://</code>,
+<code>ws://</code> (<span class="availability">since Chrome 58</span>),
+<code>wss://</code> (<span class="availability">since Chrome 58</span>), or
 <code>chrome-extension://</code>.
 In addition, even certain requests with URLs using one of the above schemes
 are hidden, e.g.,
@@ -142,6 +144,17 @@ For example, for the <q>file:</q> scheme, only <code>onBeforeRequest</code>,
 <code>onErrorOccurred</code> may be dispatched.
 </p>
 
+<p>
+<span class="availability">Starting from Chrome 58</span>, webRequest API

[Devlin, 2017/02/21 16:10:44]

nit: *the* webRequest API

[pkalinnikov, 2017/02/21 19:18:16]

Done.

+supports intercepting the WebSocket handshake request. Since the handshake is
+done by means of an HTTP upgrade request, its flow fits into HTTP-oriented
+webRequest model. Note that the API does <b>not intercept</b>:
+<ul>
+  <li>Individual messages sent over an established WebSocket connection.</li>
+  <li>WebSocket closing connection.</li>
+</ul>
+</p>

[Devlin, 2017/02/21 16:10:44]

should we also mention that redirects are not supported?

edit: I see that we do this on the method, so that's probably sufficient.  It
might still be worth calling out explicitly here, but I don't feel strongly, so
up to you.

[pkalinnikov, 2017/02/21 19:18:16]

Done.

+
 <h2 id="concepts">Concepts</h2>
 
 <p>As the following sections explain, events in the web request API use request