Add test coverage of field trial for Certificate Transparency log auditing

It is intended that this feature will be slowly turned on in the near
future. Only desktop platforms will be enabled initially.

Bug: 506227
Change-Id: Ie6669f1ecb07c84ee26f28349b30f3da5b5cbf8f
Reviewed-on: https://chromium-review.googlesource.com/873531Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Rob Percival <robpercival@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536100}

testing/variations/fieldtrial_testing_config.json

@@ -773,6 +773,24 @@
             ]
         }
     ],
+    "CertificateTransparencyLogAuditing": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CertificateTransparencyLogAuditing"
+                    ]
+                }
+            ]
+        }
+    ],
     "CheckerImaging": [
         {
             "platforms": [