Re-add MacCustomPaperSize experiment to field trial testing config

Essentially a reland of crrev.com/741291 with the correct feature name.
The original CL used a wrong and non-existent feature name.
Consequently, the config was detected as stale and was removed in
crrev.com/742702.

Bug: 1021350
Change-Id: Iee460cd6b7eac51134c8a7aba79e68e1f1fd655d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225484Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774802}

testing/variations/fieldtrial_testing_config.json

@@ -3496,6 +3496,21 @@
             ]
         }
     ],
+    "MacCustomPaperSize": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableCustomMacPaperSizes"
+                    ]
+                }
+            ]
+        }
+    ],
     "MacSystemMediaPermissionsInfoUI": [
         {
             "platforms": [