Add field trial testing config for preload=metadata suspend.

BUG=827396
TEST=passes the bots

Change-Id: Ic88e877aecdd658c734182eb21f5849f0e2363fe
Reviewed-on: https://chromium-review.googlesource.com/987195Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553017}

testing/variations/fieldtrial_testing_config.json

@@ -2652,6 +2652,25 @@
             ]
         }
     ],
+    "PreloadMetadataSuspend": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "PreloadMetadataSuspend"
+                    ]
+                }
+            ]
+        }
+    ],
     "PreviewsClientLoFi": [
         {
             "platforms": [