Add fieldtrial testing for DefaultPassthroughCommandDecoder.

BUG=602688

Change-Id: If13efb71c6d3c471c95aa2e5649e943dad25a9d7
Reviewed-on: https://chromium-review.googlesource.com/781800Reviewed-by: Gayane Petrosyan <gayane@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519793}

testing/variations/fieldtrial_testing_config.json

@@ -1143,6 +1143,21 @@
             ]
         }
     ],
+    "DefaultPassthroughCommandDecoder": [
+        {
+            "platforms": [
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "DefaultPassthroughCommandDecoder",
+                    "enable_features": [
+                        "DefaultPassthroughCommandDecoder"
+                    ]
+                }
+            ]
+        }
+    ],
     "DelayNavigation": [
         {
             "platforms": [