Add test config for UseInProgressDownloadManagerForDownloadService

BUG=1003904

Change-Id: Ibce58e04c2d0fa21549ebbb060a5bcd43f9b8d2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914820Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715931}

testing/variations/fieldtrial_testing_config.json

@@ -6877,6 +6877,21 @@
             ]
         }
     ],
+    "UseInProgressDownloadManagerForDownloadService": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "UseInProgressDownloadManagerForDownloadService"
+                    ]
+                }
+            ]
+        }
+    ],
     "UseSkiaRenderer": [
         {
             "platforms": [