Add a field trial testing config for ViewsCastDialog

Launch bug: http://crbug.com/754101
The plan is to launch the feature in M71.

Change-Id: Idde08df84304ad6f2668603579fdc7e619072892
Reviewed-on: https://chromium-review.googlesource.com/c/1262656Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598935}

testing/variations/fieldtrial_testing_config.json

@@ -4992,6 +4992,24 @@
             ]
         }
     ],
+    "ViewsCastDialog": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "ViewsCastDialog"
+                    ]
+                }
+            ]
+        }
+    ],
     "VizDisplayCompositor": [
         {
             "platforms": [