Add fieldtrial testing experiment for enabling browser side DIAL discovery

We are turning on 50% on beta. Got a finch warning that we must add a field trial config for it.

Bug: 718154
Change-Id: I480d390acf1f9f903a00345169d270d6ca01b428
Reviewed-on: https://chromium-review.googlesource.com/596627Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Commit-Queue: Bin Zhao <zhaobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491164}

testing/variations/fieldtrial_testing_config.json

@@ -816,6 +816,24 @@
             ]
         }
     ],
+    "EnableDialLocalDiscovery": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableDialLocalDiscovery"
+                    ]
+                }
+            ]
+        }
+    ],
     "EnableGoogleCachedCopyTextExperiment": [
         {
             "platforms": [