Add testing configs for ParseHTMLOnMainThread experiment

testing/variations/fieldtrial_testing_config_linux.json

 {
     "AsyncDNS": [
         {
             "group_name": "AsyncDNSA"
         }
     ],
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
@@ skipping 137 matching lines @@
     "OutOfProcessPac": [
         {
             "group_name": "Enabled"
         }
     ],
     "PageRevisitInstrumentation": [
         {
             "group_name": "Enabled"
         }
     ],
+    "ParseHTMLOnMainThread": [
+        {
+            "enable_features": [
+                "ParseHTMLOnMainThread"
+            ],
+            "group_name": "Enabled",
+            "params": {
+                "parseHTMLOnMainThreadCoalesceChunks": "false",
+                "parseHTMLOnMainThreadSyncTokenize": "false"
+            }
+        }
+    ],
     "PassiveDocumentEventListeners": [
         {
             "enable_features": [
                 "PassiveDocumentEventListeners"
             ],
             "group_name": "Enabled"
         }
     ],
     "PasswordBranding": [
         {
@@ skipping 218 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }