Hide share button if getDisplayMedia doesn't exist.

static/sfu.css

@@ -1013,10 +1013,6 @@ header .collapse {
         border-right: none;
     }
 
-    #sharebutton {
-        display: none;
-    }
-
     .dropdown-content {
         margin-top: 10px;
     }

static/sfu.js

@@ -376,11 +376,14 @@ function setButtonsVisibility() {
     let permissions = serverConnection.permissions;
     let local = !!findUpMedia('local');
     let share = !!findUpMedia('screenshare')
+
     // don't allow multiple presentations
     setVisibility('presentbutton', permissions.present && !local);
     setVisibility('unpresentbutton', local);
+
     // allow multiple shared documents
-    setVisibility('sharebutton', permissions.present);
+    setVisibility('sharebutton', permissions.present &&
+                  ('getDisplayMedia' in navigator.mediaDevices))
     setVisibility('unsharebutton', share);
 
     setVisibility('mediaoptions', permissions.present);