[profiler] Start timer when profiling to get timestamps in profview.

Bug: v8:6520
Change-Id: I3d5a019885516d7cbc4f43bfb3892bed655a9628
Reviewed-on: https://chromium-review.googlesource.com/586708Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46901}

src/log.cc

@@ -1774,7 +1774,7 @@ bool Logger::SetUp(Isolate* isolate) {
     is_logging_ = true;
   }
 
-  if (FLAG_log_internal_timer_events) timer_.Start();
+  if (FLAG_log_internal_timer_events || FLAG_prof_cpp) timer_.Start();
 
   if (FLAG_prof_cpp) {
     profiler_ = new Profiler(isolate);