Add MacMDDownloadShelf to fieldtrial_testing_config.json

Change-Id: I00799bf180fc94b3a8b30e0a77bdc7473cf58ca4
Bug: 589943
Reviewed-on: https://chromium-review.googlesource.com/887385Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532044}

testing/variations/fieldtrial_testing_config.json

@@ -1852,6 +1852,21 @@
             ]
         }
     ],
+    "MacMDDownloadShelf": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "MacMDDownloadShelf"
+                    ]
+                }
+            ]
+        }
+    ],
     "MacV2Sandbox": [
         {
             "platforms": [