Add CertDualVerificationTrial to fieldtrial_testing_config.

Change-Id: I8ec1995b62eebedc4e2ea873399168fb73484dd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536691Reviewed-by: Steven Valdez <svaldez@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Steven Valdez <svaldez@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644413}

testing/variations/fieldtrial_testing_config.json

@@ -1250,6 +1250,22 @@
             ]
         }
     ],
+    "CertDualVerificationTrial": [
+        {
+            "platforms": [
+                "linux",
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CertDualVerificationTrial"
+                    ]
+                }
+            ]
+        }
+    ],
     "CertificateTransparencyLogAuditing": [
         {
             "platforms": [