Add fieldtrial config for CompositeCrossOriginIframes

BUG=1060884

Change-Id: If988b891883e9e5ce1d6ba56695a84b149803eb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368452Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801892}

testing/variations/fieldtrial_testing_config.json

@@ -1309,6 +1309,27 @@
             ]
         }
     ],
+    "CompositeCrossOriginIframes": [
+        {
+            "platforms": [
+                "android",
+                "android_weblayer",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CompositeCrossOriginIframes"
+                    ]
+                }
+            ]
+        }
+    ],
     "CompositingOptimizations": [
         {
             "platforms": [