Video Capture Mojo (1.4a.a): Add service configurator interface

services/video_capture/public/interfaces/service_configurator.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 video_capture.mojom;
+
+import "services/video_capture/public/interfaces/video_capture_device_factory.mojom";
+
+// Allows clients to add fake and file-based video capture devices to be offered
+// through the VideoCaptureDeviceFactory instance exposed by the service.
+interface ServiceConfigurator {

[mcasas, 2016/08/12 23:38:37]

If this is just for testing (and it is: currently users need
to add command-line flags for Fake and File VCDs), I
think the mojom convention is to add Test to the name,
e.g. ipc/ipc_test.mojom, url/mojo/url_test.mojom adn others.

[chfremer, 2016/08/15 20:10:34]

Good point. I am not sure, though, if this is really considered test-only
"enough". As you pointed out in the other comment, the fake devices are actually
shipped in production builds, so managing those devices can be considered legit
production functionality. I'll keep this in mind, but since I am proposing to
change the structure of the interface a little bit in the next PatchSet, this
may not apply anymore. Please see next PatchSet.

+  // Returns the descriptor under which the newly added fake device will be
+  // advertised in VideoCaptureDeviceFactory::EnumerateDeviceDescriptors().
+  AddFakeVideoCaptureDevice()

[mcasas, 2016/08/12 23:38:37]

A comment: Fake and File VCDs are used a lot for
our own testing in bots that sometimes have working,
but inadequate, capture devices; for example, the CQ
linux_android_rel_ng uses an Android emulator that has
two cameras, but that don't work well for 
content_browsertests. So: Fake and File do not coexist
with the eventual 'real' webcams, but eclipse them
altogether. I think we should try to repro that behaviour
here as well.

[chfremer, 2016/08/15 20:10:34]

Agreed that we probably do not need to have a single factory that mixes both
real and fake devices. After thinking about it for a bit, instead of exposing
switch to switch the factory between real and fake, I propose to have the
service expose two factory instances, one for the actual devices, and one for
fake devices. A separate interface is exposed for configuration of the fake
factory. WDYT?

+      => (VideoCaptureDeviceDescriptor descriptor);
+};