CrOS GMC: Add field trail entry for experiment.

Change-Id: Ic832f0e49123cff85fe5ab8e0dd12e43a9c38db1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493094
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832908}

testing/variations/fieldtrial_testing_config.json

@@ -3171,6 +3171,24 @@
             ]
         }
     ],
+    "GlobalMediaControlsForChromeOS": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "EnabledWithDefaultPinnedv1",
+                    "params": {
+                        "CrosGlobalMediaControlsPinParam": "default-pinned"
+                    },
+                    "enable_features": [
+                        "GlobalMediaControlsForChromeOS"
+                    ]
+                }
+            ]
+        }
+    ],
     "GlobalMediaControlsInProductHelp": [
         {
             "platforms": [