Fix handling time counters

Summary:
Currently we'll are getting incorrect Idle&Busy time counters for loops when there were no LoopCallbacks. This is because we don't properly refresh the `startWork_` and use the one from the last loop that had LoopCallbacks.

This diff fixes that by properly refreshing `startWork_` even if there're no LoopCallbacks.

Reviewed By: yfeldblum

Differential Revision: D7811127

fbshipit-source-id: 94ded08963b1bddc7b7859484f076d4c8a1e897f

folly/io/async/EventBase.cpp

@@ -592,8 +592,8 @@ bool EventBase::runImmediatelyOrRunInEventBaseThreadAndWait(Func fn) {
 }
 
 bool EventBase::runLoopCallbacks() {
+  bumpHandlingTime();
   if (!loopCallbacks_.empty()) {
-    bumpHandlingTime();
     // Swap the loopCallbacks_ list with a temporary list on our stack.
     // This way we will only run callbacks scheduled at the time
     // runLoopCallbacks() was invoked.