Enable WebRTC-VP8VariableFramerateScreenshare on perfbots

Bug: chromium:949112
Change-Id: I33b42200891e72bc00cfc44933d312ce918f8839
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362287
Auto-Submit: Ilya Nikolaevskiy <ilnik@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799133}

testing/variations/fieldtrial_testing_config.json

@@ -7870,6 +7870,21 @@
             ]
         }
     ],
+    "WebRTC-VP8VariableFramerateScreenshare": [
+        {
+            "platforms": [
+                "windows",
+                "mac",
+                "chromeos",
+                "linux"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled"
+                }
+            ]
+        }
+    ],
     "WebRTC-Video-MinVideoBitrate": [
         {
             "platforms": [