Update fieldtrial_testing_config.json for OffMainThreadFetch

Bug: 443374
Change-Id: I001c3981e17430f8786eec2a7d475c0c994e2f5e
Reviewed-on: https://chromium-review.googlesource.com/597573Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491557}

testing/variations/fieldtrial_testing_config.json

@@ -1732,6 +1732,31 @@
             ]
         }
     ],
+    "OffMainThreadFetch": [
+        {
+            "platforms": [
+                "android",
+                "chromeos",
+                "linux",
+                "mac",
+                "win"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "OffMainThreadFetch"
+                    ]
+                },
+                {
+                    "name": "Disabled",
+                    "disable_features": [
+                        "OffMainThreadFetch"
+                    ]
+                }
+            ]
+        }
+    ],
     "OfferUploadCreditCards": [
         {
             "platforms": [