Download later: Enable the feature in field trial config.

This CL adds download later feature to fild trial json config file.

TBR=mpearson@chromium.org

Bug: NONE
Change-Id: I0de7dcb2afeb796ac02083a65a8cf6ee2f205d97
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438576
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812140}

testing/variations/fieldtrial_testing_config.json

@@ -2466,6 +2466,7 @@
                 {
                     "name": "Enabled",
                     "enable_features": [
+                        "DownloadLater",
                         "DownloadsAutoResumptionNative"
                     ]
                 }