[iOS] FieldTrial Testing Config for Full Page Screenshots

Bug: 1103778
Change-Id: Id4fa1e2ff13e376bfd8d8c99e7405793375225d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422179Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808925}

testing/variations/fieldtrial_testing_config.json

@@ -3444,6 +3444,21 @@
             ]
         }
     ],
+    "IOSFullPageScreenshots": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableFullPageScreenshot"
+                    ]
+                }
+            ]
+        }
+    ],
     "IOSLegacyTLSInterstitial": [
         {
             "platforms": [