Add CupsIppPrintingBackend experiment to field trial testing config

Bug: 226176
Change-Id: Ia36e7a0e0c3334c8b45dfe3b0036a84719dea12c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278223Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785905}

testing/variations/fieldtrial_testing_config.json

@@ -1901,6 +1901,21 @@
             ]
         }
     ],
+    "CupsIppPrintingBackend": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CupsIppPrintingBackend"
+                    ]
+                }
+            ]
+        }
+    ],
     "D3D11VideoDecoder": [
         {
             "platforms": [