Handle gUM failures on FF when no audio or video device is available.

Since FF 49, the call to gUM only succeeds if both audio and video constraints
can be satisfied, i.e. without a webcam the request fails. This is now handled
gracefully, the available devices enumerated and the gUM request is retried
with updated constraints (fixes #394).

static/js/mediastream/usermedia.js

@@ -98,7 +98,52 @@ define(['jquery', 'underscore', 'audiocontext', 'mediastream/dummystream', 'webr
 				var c = {audio: convertConstraints(constraints.audio), video: convertConstraints(constraints.video)};
 				// mediaDevices API returns a promise.
 				console.log("Constraints for mediaDevices", c);
-				window.navigator.mediaDevices.getUserMedia(c).then(success).catch(error);
+				window.navigator.mediaDevices.getUserMedia(c).then(success).catch(function(err) {
+					if (!navigator.mediaDevices.enumerateDevices) {
+						// Don't know how to check for available devices.
+						error(err);
+						return;
+					}
+
+					// gUM fails if one of audio/video is not available, check which devices are
+					// available and retry with updated constraints.
+					console.log("getUserMedia with audio/video contraints failed", err);
+					navigator.mediaDevices.enumerateDevices().then(function(devices) {
+						var has_audio = false;
+						var has_video = false;
+						console.log("Available devices", devices);
+						_.each(devices, function(device) {
+							switch (device.kind) {
+								case "audioinput":
+									has_audio = true;
+									break;
+								case "videoinput":
+									has_video = true;
+									break;
+								default:
+									break;
+							}
+						});
+						if (!has_audio && !has_video) {
+							// No audio or video device found, no need to retry gUM.
+							error(err);
+							return;
+						}
+
+						if (!has_audio) {
+							delete c.audio;
+						}
+						if (!has_video) {
+							delete c.video;
+						}
+						console.log("Retry getUserMedia with updated constraints", c);
+						window.navigator.mediaDevices.getUserMedia(c).then(success).catch(error);
+					}).catch(function(devicesError) {
+						console.log("Could not enumerate devices", devicesError);
+						// Fail initial gUM
+						error(err);
+					});
+				});
 			}
 		} else {
 			// Use existing adapter.