[ios] Add testing config for DragAndDrop

Bug: 1099646
Change-Id: I2b88eb75acc48c02d58b3febdea380410a74861f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2321213
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792153}

testing/variations/fieldtrial_testing_config.json

@@ -3422,6 +3422,21 @@
             ]
         }
     ],
+    "IOSDragAndDrop": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "DragAndDrop"
+                    ]
+                }
+            ]
+        }
+    ],
     "IOSEnablePersistentDownloads": [
         {
             "platforms": [