[ios] Add testing config for AddWebContentDropInteraction

Bug: 1099646
Change-Id: Icb8adf3f1b599c9a8cb12dd7cee563432db5671a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2442171
Auto-Submit: edchin <edchin@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813951}

testing/variations/fieldtrial_testing_config.json

@@ -3400,6 +3400,21 @@
             ]
         }
     ],
+    "IOSWebContentDropInteraction": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "AddWebContentDropInteraction"
+                    ]
+                }
+            ]
+        }
+    ],
     "Identifiability": [
         {
             "platforms": [