Add simulcast screenshare experiment configuration

Bug: 690537
Change-Id: Ie75bd4040aee327431b01c676a221a8eb70410ae
Reviewed-on: https://chromium-review.googlesource.com/1106146Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568478}

testing/variations/fieldtrial_testing_config.json

@@ -4643,6 +4643,21 @@
             ]
         }
     ],
+    "WebRTC-SimulcastScreenshare": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled"
+                }
+            ]
+        }
+    ],
     "WebRtcUseEchoCanceller3": [
         {
             "platforms": [