Plumb experiment flags through page_load_metrics and add a new observer

testing/variations/fieldtrial_testing_config_linux.json

 {
     "AsyncDNS": [
         {
             "group_name": "AsyncDNSA"
         }
     ],
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
@@ skipping 31 matching lines @@
         {
             "group_name": "Enabled"
         }
     ],
     "DocumentWriteEvaluator": [
         {
             "enable_features": [
                 "DocumentWriteEvaluator"
             ],
             "group_name": "Enabled"
+        },
+        {
+            "group_name": "Default"
         }
     ],
     "EnableGoogleCachedCopyTextExperiment": [
         {
             "group_name": "Button"
         }
     ],
     "EnableSessionCrashedBubbleUI": [
         {
             "group_name": "Enabled"
@@ skipping 197 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }