Ensure/document that the first parameter to the native messaging host is the origin

chrome/common/extensions/docs/templates/articles/nativeMessaging.html

 <h1>Native Messaging</h1>
 <p>
 Extensions and apps can exchange messages with native applications using an API
 that is similar to the other <a href="messaging">message passing APIs</a>.
 Native applications that support this feature must register a
 <em>native messaging host</em> that knows how to communicate with the extension.
 Chrome starts the host in a separate process and communicates with it using
 standard input and standard output streams.
 
 <h2 id="native-messaging-host">Native messaging host</h2>
@@ skipping 111 matching lines @@
 Chrome starts each native messaging host in a separate process and communicates
 with it using standard input (<code>stdin</code>) and standard output
 (<code>stdout</code>). The same format is used to send messages in both
 directions: each message is serialized using JSON, UTF-8 encoded
 and is preceded with 32-bit message length in native byte order.
 The maximum size of a single message from the native messaging host is 1 MB,
 mainly to protect Chrome from misbehaving native applications. The maximum
 size of the message sent to the native messaging host is 4 GB.
 
 <p>
+The first argument to the native messaging host is the origin of the caller,
+usually <code>chrome-extension://[extension ID of whitelisted extension]</code>.

[Devlin, 2016/09/26 15:53:59]

nit: s/extension ID of whitelisted extension/ID of the whitelisted extension

[robwu, 2016/09/28 09:07:09]

Done.

+This allows native messaging hosts to identify the source of the message when it
+has allowed multiple extensions to use the native messaging host via the
+<code>allowed_origins</code> key in the
+<a href="#native-messaging-host-manifest">native messaging host manifest</a>.

[Devlin, 2016/09/26 15:53:59]

This sentence reads a little funny.  Maybe

"This allows native messaging hosts to identify the source of the message when
multiple extensions are specified in the <code>allowed_origins</code> key in the
<a href="#native-messaging-host-manifest">host manifest</a>."

WDYT?

[robwu, 2016/09/28 09:07:09]

SGTM. I used "native messaging host manifest" instead of "host manifest" to
emphasize that it is not manifest.json but the manifest of the NMH.

+
+<p>
 When a messaging port is created using $(ref:runtime.connectNative) Chrome
 starts native messaging host process and keeps it running until the port is
 destroyed. On the other hand, when a message is sent using
 $(ref:runtime.sendNativeMessage), without creating a messaging port, Chrome starts
 a new native messaging host process for each message. In that case the first
 message generated by the host process is handled as a response to the original
 request, i.e. Chrome will pass it to the response callback specified when
 $(ref:runtime.sendNativeMessage) is called. All other messages generated by the
 native messaging host in that case are ignored.
 
@@ skipping 134 matching lines @@
 and
 <a href="https://developer.chrome.com/extensions/examples/api/nativeMessaging/host.zip">sample host</a>.
 
 Run <code>install_host.bat</code> (Windows) or
 <code>install_host.sh</code> (Linux / OS X) to install the native messaging
 host.
 Then <a href="getstarted#unpacked">load the app</a> and interact with the app.
 Run <code>uninstall_host.bat</code> or <code>uninstall_host.sh</code> to
 unregister the native messaging host when you are done.