Added entry for UMAHttpRetry field trial

Bug: 766233
Change-Id: I0af9b95727cd334af60f6e50904a9d04339bd7da
Reviewed-on: https://chromium-review.googlesource.com/923181Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537343}

testing/variations/fieldtrial_testing_config.json

@@ -3920,6 +3920,26 @@
             ]
         }
     ],
+    "UMAHttpRetry": [
+        {
+            "platforms": [
+                "win",
+                "mac",
+                "chromeos",
+                "linux",
+                "ios",
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "UMAHttpRetryEnabled",
+                    "enable_features": [
+                        "UMAHttpRetry"
+                    ]
+                }
+            ]
+        }
+    ],
     "UpdateMenuItem": [
         {
             "platforms": [