Test that page performance metrics are retrieved and the list is stable.
DO NOT modify the list, it's exposed over public protocol.

Metrics:


Metrics:
Documents
Frames
JSEventListeners
JSHeapTotalSize
JSHeapUsedSize
LayoutCount
LayoutDuration
Nodes
RecalcStyleCount
RecalcStyleDuration
ScriptDuration
TaskDuration
Timestamp

Metrics: