Avoid a superfluous call to ProcessEvents in MessagePumpKqueue

Change-Id: I0031a90c2bd4ae21c42fe7e38a411186b270e6b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2822898
Reviewed-by: François Doray <fdoray@chromium.org>
Commit-Queue: Olivier Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#872449}

base/message_loop/message_pump_kqueue.cc

@@ -390,6 +390,11 @@ bool MessagePumpKqueue::DoInternalWork(Delegate* delegate,
                                  events_.size(), flags, nullptr));
 
   PCHECK(rv >= 0) << "kevent64";
+  if (rv == 0) {
+    // No events to dispatch so no need to call ProcessEvents().
+    return false;
+  }
+
   return ProcessEvents(delegate, rv);
 }