Mark inspector/network/network-domain-filter.html as slow on Mac and XP

BUG=451577
TBR=pfeldman@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1550143003

Cr-Commit-Position: refs/heads/master@{#367002}

third_party/WebKit/LayoutTests/SlowTests

@@ -81,6 +81,7 @@ crbug.com/451577 [ Mac XP ] inspector/extensions/extensions-reload.html [ Slow ]
 crbug.com/451577 [ Mac XP ] inspector/extensions/extensions-resources.html [ Slow ]
 crbug.com/451577 [ Win10 XP ] inspector/extensions/extensions-sidebar.html [ Slow ]
 crbug.com/451577 [ Mac XP ] inspector/layers/layer-canvas-log.html [ Slow ]
+crbug.com/451577 [ Mac XP ] inspector/network/network-domain-filter.html [ Slow ]
 
 crbug.com/474121 editing/deleting/password-delete-performance.html [ Slow ]
 crbug.com/237245 [ Debug Win7 ] editing/selection/programmatic-selection-on-mac-is-directionless.html [ Slow ]