Add DataReductionProxyBrotliHoldback to field trial testing config

Bug: 891458
Change-Id: I144a1eb2c528b0e02f45a7b000628c07b1ba78c3
Reviewed-on: https://chromium-review.googlesource.com/c/1287893Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600942}

testing/variations/fieldtrial_testing_config.json

@@ -1328,6 +1328,25 @@
             ]
         }
     ],
+    "DataReductionProxyBrotliHoldback": [
+        {
+            "platforms": [
+                "android",
+                "windows",
+                "mac",
+                "chromeos",
+                "linux"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "DataReductionProxyBrotliHoldback"
+                    ]
+                }
+            ]
+        }
+    ],
     "DataReductionProxyFREPromo": [
         {
             "platforms": [