Add a fieldtrial testing config for Mach SharedMemory experiment.

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 63 matching lines @@
     "InstanceID": [
         {
             "group_name": "Enabled"
         }
     ],
     "IntelligentSessionRestore": [
         {
             "group_name": "Enabled"
         }
     ],
+    "MacMemoryMechanism": [
+        {
+            "group_name": "Mach"
+        }
+    ],
     "MaterialDesignDownloads": [
         {
             "group_name": "Enabled"
         }
     ],
     "PasswordBranding": [
         {
             "group_name": "SmartLockBrandingSavePromptOnly"
         }
     ],
@@ skipping 117 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }