Add EnableCastLocalMedia to Finch field test config

Launch bug: crbug.com/721802
Finch config: cl/187885128, cl/189269157

Change-Id: I47a5c66de97b312ed5931ec83f94a2c62f8d850d
Reviewed-on: https://chromium-review.googlesource.com/965388Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Adam Parker <amp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543591}

testing/variations/fieldtrial_testing_config.json

@@ -1303,6 +1303,24 @@
             ]
         }
     ],
+    "EnableCastLocalMedia": [
+        {
+            "platforms": [
+                "linux",
+                "mac",
+                "win",
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableCastLocalMedia"
+                    ]
+                }
+            ]
+        }
+    ],
     "EnableCsrssLockdown": [
         {
             "platforms": [