Add testing config for OverlayFullscreenVideo.

testing/variations/fieldtrial_testing_config_android.json

 {
     "AllBookmarks": [
         {
             "disable_features": [
                 "AllBookmarks"
             ],
             "group_name": "Disabled"
         }
     ],
     "AndroidSessionNotifications": [
@@ skipping 282 matching lines @@
         }
     ],
     "OfflinePages": [
         {
             "enable_features": [
                 "NTPOfflinePages"
             ],
             "group_name": "EnabledAsBookmarks"
         }
     ],
+    "OverlayFullscreenVideo": [
+        {
+            "disable_features": [
+                "overlay-fullscreen-video"
+            ],
+            "group_name": "Disabled"
+        }
+    ],
     "PageRevisitInstrumentation": [
         {
             "group_name": "Enabled"
         }
     ],
     "ParseHTMLOnMainThread": [
         {
             "enable_features": [
                 "ParseHTMLOnMainThread"
             ],
@@ skipping 280 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }