[iOS] Adds EnableCloseAllTabsConfirmation config

This CL adds the EnableCloseAllTabsConfirmation feature to the
fieldtrial testing config.

Bug: 1119319
Change-Id: I197528caa2d090e2ddd6fde43b18223da49e226f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513597
Commit-Queue: Ewann Pellé <ewannpv@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824884}

testing/variations/fieldtrial_testing_config.json

@@ -2562,6 +2562,21 @@
             ]
         }
     ],
+    "EnableCloseAllTabsConfirmation": [
+        {
+            "platforms": [
+                "ios"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableCloseAllTabsConfirmation"
+                    ]
+                }
+            ]
+        }
+    ],
     "EnableContextMenuPerformanceHints": [
         {
             "platforms": [