[Media History] Add field trial testing conf

Add field trial testing conf since we plan
to launch this through finch.

BUG=1024351

Change-Id: I7912c1733f453fd43222f7fd2908d4120aaa3476
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2140196
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762131}

testing/variations/fieldtrial_testing_config.json

@@ -3306,6 +3306,25 @@
             ]
         }
     ],
+    "MediaHistory": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "UseMediaHistoryStore"
+                    ]
+                }
+            ]
+        }
+    ],
     "MergeBlockingNonBlockingPools": [
         {
             "platforms": [