Enable GPU Raster for Mac waterfall

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 34 matching lines @@
         }
     ],
     "ChromotingQUIC": [
         {
             "group_name": "Enabled",
             "params": {
                 "delay_tcp_race": "true"
             }
         }
     ],
+    "DefaultEnableGpuRasterization": [
+        {
+            "enable_features": [
+                "DefaultEnableGpuRasterization"
+            ],
+            "group_name": "DefaultEnableGpuRasterization"
+        }
+    ],
     "DisallowFetchForDocWrittenScriptsInMainFrame": [
         {
             "disable_features": [
                 "DocumentWriteEvaluator"
             ],
             "group_name": "DocumentWriteScriptBlockGroup",
             "params": {
                 "disallowFetchForDocWrittenScriptsInMainFrameOnSlowConnections": "true"
             }
         }
@@ skipping 339 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }