Add a fieldtrial testing config for Mach SharedMemory experiment.

Note that Mach shared memory is already turned on by default on trunk, so this
has no effect. The main purpose of this CL is to disable a Finch warning. See
https://critique.corp.google.com/#review/113054955 for more details.

BUG=552422

Review URL: https://codereview.chromium.org/1637063002

Cr-Commit-Position: refs/heads/master@{#371557}

testing/variations/fieldtrial_testing_config_mac.json

@@ -81,6 +81,11 @@
             "group_name": "Enabled"
         }
     ],
+    "MacMemoryMechanism": [
+        {
+            "group_name": "Mach"
+        }
+    ],
     "MaterialDesignDownloads": [
         {
             "group_name": "Enabled"