Restrict VP9 experiment to Chrome >= 48.

static/js/services/constraints.js

@@ -202,6 +202,7 @@
 					hdVideo: isChrome || (isFirefox && version >= 38),
 					// Chrome supports this on Windows only.
 					renderToAssociatedSink: isChrome && $window.navigator.platform.indexOf("Win") === 0,
+					vp9: isChrome && version >= 48,
 					chrome: isChrome,
 					firefox: isFirefox,
 					edge: isEdge

static/partials/settings.html

@@ -270,7 +270,7 @@
               </div>
             </div>
 
-            <div class="form-group">
+            <div class="form-group" ng-show="supported.constraints.vp9">
               <label class="col-xs-4 control-label">{{_('Prefer VP9 video codec')}}</label>
               <div class="col-xs-8">
                 <div class="checkbox">