Add chrome://usb-internals page for adding and removing test devices.

chrome/browser/ui/webui/usb_internals/usb_internals.mojom

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module mojom;
+
+struct TestDeviceInfo {
+  string guid;
+  string name;
+  string serial_number;
+  string landing_page;
+  string allowed_origin;
+};
+
+interface UsbInternalsPageHandler {
+  // Simulate the connection of a new device with the given properties.
+  AddDeviceForTesting(string name,
+                      string serial_number,
+                      string landing_page,
+                      string allowed_origin) => (bool success, string message);

[dpapad, 2016/08/04 20:55:34]

Ideally we would want the Promise that JS holds to be rejected instead of
resolved, in the case of an error.

addDeviceForTesting(...).then(
   function(message) { // handles only the success case },
   function(error) { // handles only the error case }); // no need for |success|
boolean.

Instead of passing a |success| boolean and handling both success/error in the
success callback. I am not sure if there is currently a way to reject a Promise
from the Mojo-based C++ handler. Perhaps worth asking on chromium-mojo, since
this seems it will come up a lot in the future.

[Reilly Grant (use Gerrit), 2016/08/05 01:30:55]

There is not, because it doesn't translate well to C++ there is no concept of
failure in Mojo calls.

+
+  // Simulate the disconnection of a device added with the function above.
+  RemoveDeviceForTesting(string guid) => ();

[dpapad, 2016/08/04 20:55:34]

It is not clear where the guid is found. Based on the comment here, I was
expecting that |guid| is returned from a call to AddDeviceForTesting, and then
it can be used to remove that device. Looking at the JS code, I am realizing
that AddDeviceForTesting, does not return the |guid|, instead a call to
GetTestDevices() is necessary.

How about changing the signature of AddDeviceForTesting as follows?

AddDeviceForTesting(string name,
                    string serial_number,
                    string landing_page,
                    string allowed_origin) => (bool success, TestDeviceInfo
device);

Or if we figure out the reject/resolve issue mentioned in the previous comment,
this could be even simpler

AddDeviceForTesting(string name,
                    string serial_number,
                    string landing_page,
                    string allowed_origin) => (TestDeviceInfo device);

[Reilly Grant (use Gerrit), 2016/08/05 01:30:55]

It simplifies the JS code if it doesn't have to handle adding and removing
individual entries and just receives the full list via Mojo each time.

+
+  // Retrieves the list of test devices added with this API.
+  GetTestDevices() => (array<TestDeviceInfo> devices);
+};