feat(external_api) allow initial gUM requests to be disabled

conference.js

@@ -472,8 +472,8 @@ export default {
      */
     createInitialLocalTracks(options = {}) {
         const errors = {};
-        const initialDevices = [ 'audio' ];
+        const initialDevices = config.disableInitialGUM ? [] : [ 'audio' ];
-        const requestedAudio = true;
+        const requestedAudio = !config.disableInitialGUM;
         let requestedVideo = false;
 
         // Always get a handle on the audio input device so that we have statistics even if the user joins the
@@ -484,19 +484,22 @@ export default {
             this.muteAudio(true, true);
         }
 
-        if (!options.startWithVideoMuted
+        if (!config.disableInitialGUM
+                && !options.startWithVideoMuted
                 && !options.startAudioOnly
                 && !options.startScreenSharing) {
             initialDevices.push('video');
             requestedVideo = true;
         }
 
+        if (!config.disableInitialGUM) {
             JitsiMeetJS.mediaDevices.addEventListener(
                 JitsiMediaDevicesEvents.PERMISSION_PROMPT_IS_SHOWN,
                 browserName =>
                     APP.store.dispatch(
                         mediaPermissionPromptVisibilityChanged(true, browserName))
             );
+        }
 
         let tryCreateLocalTracks;
 

config.js

@@ -336,6 +336,10 @@ var config = {
     // will be joined when no room is specified.
     enableWelcomePage: true,
 
+    // Disable initial browser getUserMedia requests.
+    // This is useful for scenarios where users might want to start a conference for screensharing only
+    // disableInitialGUM: false,
+
     // Enabling the close page will ignore the welcome page redirection when
     // a call is hangup.
     // enableClosePage: false,

react/features/base/config/configWhitelist.js

@@ -80,6 +80,7 @@ export default [
     'disableAP',
     'disableAudioLevels',
     'disableDeepLinking',
+    'disableInitialGUM',
     'disableH264',
     'disableHPF',
     'disableInviteFunctions',