Use the pause event's topFrame to satisfy initial top-frame request #2974

When the debugger pauses and we get a pauseEvent, it contains the topFrame of the stack. We currently ignore this, and when asked for the call stack, send a getStack() request to the VM.

VS Code asks us first for the top frame, and then for the next 20. We could save a round trip to the VM (and causing two concurrent getStack() requests) when the stackTraceRequest is for only the top frame and the current pauseEvent for the isolate contains a topFrame.