variations: Also enable the RenderingPipelineThrottling feature

testing/variations/fieldtrial_testing_config_linux.json

 {
     "AsyncDNS": [
         {
             "group_name": "AsyncDNSA"
         }
     ],
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
@@ skipping 110 matching lines @@
             "group_name": "Enabled"
         }
     ],
     "RefreshTokenDeviceId": [
         {
             "group_name": "Enabled"
         }
     ],
     "RenderingPipelineThrottling": [
         {
+            "enable_features": [
+                "RenderingPipelineThrottling"
+            ],
             "group_name": "Enabled"
         }
     ],
     "ReportCertificateErrors": [
         {
             "group_name": "ShowAndPossiblySend",
             "params": {
                 "sendingThreshold": "1.0"
             }
         },
@@ skipping 122 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }