electron: call into JitsiMeet to open a desktop picker

#416

JitsiTrackError.js

     = 'User canceled Chrome\'s screen sharing prompt';
 TRACK_ERROR_TO_MESSAGE_MAP[JitsiTrackErrors.CHROME_EXTENSION_GENERIC_ERROR]
     = 'Unknown error from Chrome extension';
+TRACK_ERROR_TO_MESSAGE_MAP[JitsiTrackErrors.ELECTRON_DESKTOP_PICKER_ERROR]
+    = 'Unkown error from desktop picker';
+TRACK_ERROR_TO_MESSAGE_MAP[JitsiTrackErrors.ELECTRON_DESKTOP_PICKER_NOT_FOUND]
+    = 'Failed to detect desktop picker';
 TRACK_ERROR_TO_MESSAGE_MAP[JitsiTrackErrors.GENERAL]
     = 'Generic getUserMedia error';
 TRACK_ERROR_TO_MESSAGE_MAP[JitsiTrackErrors.PERMISSION_DENIED]

JitsiTrackErrors.js

  */
 export const CONSTRAINT_FAILED = 'gum.constraint_failed';
 
+/**
+ * A generic error which indicates an error occurred while selecting
+ * a DesktopCapturerSource from the electron app.
+ */
+export const ELECTRON_DESKTOP_PICKER_ERROR
+    = 'gum.electron_desktop_picker_error';
+
+/**
+ * An error which indicates a custom desktop picker could not be detected
+ * for the electron app.
+ */
+export const ELECTRON_DESKTOP_PICKER_NOT_FOUND
+    = 'gum.electron_desktop_picker_not_found';
+
 /**
  * An error which indicates that the jidesha extension for Firefox is
  * needed to proceed with screen sharing, and that it is not installed.

modules/RTC/ScreenObtainer.js

                     });
             };
         } else if (RTCBrowserType.isElectron()) {
-            obtainDesktopStream = (_, onSuccess, onFailure) =>
-                window.JitsiMeetElectron.obtainDesktopStream(
-                    streamId =>
-                        onGetStreamResponse({ streamId }, onSuccess, onFailure),
-                    err => onFailure(new JitsiTrackError(
-                        JitsiTrackErrors.CHROME_EXTENSION_GENERIC_ERROR, err))
-                );
+            obtainDesktopStream = (_, onSuccess, onFailure) => {
+                if (window.JitsiMeetScreenObtainer
+                    && window.JitsiMeetScreenObtainer.openDesktopPicker) {
+                    window.JitsiMeetScreenObtainer.openDesktopPicker(
+                        streamId =>
+                            onGetStreamResponse({ streamId },
+                            onSuccess,
+                            onFailure
+                        ),
+                        err => onFailure(new JitsiTrackError(
+                            JitsiTrackErrors.ELECTRON_DESKTOP_PICKER_ERROR,
+                            err
+                        ))
+                    );
+                } else {
+                    onFailure(new JitsiTrackError(
+                        JitsiTrackErrors.ELECTRON_DESKTOP_PICKER_NOT_FOUND));
+                }
+            };
         } else if (RTCBrowserType.isTemasysPluginUsed()) {
             // XXX Don't require Temasys unless it's to be used because it
             // doesn't run on React Native, for example.