Add trace event for phantom handle processing callback.

BUG=chromium:608312
LOG=NO

Review-Url: https://codereview.chromium.org/1934993002
Cr-Commit-Position: refs/heads/master@{#35933}

src/global-handles.cc

@@ -551,6 +551,7 @@ class GlobalHandles::PendingPhantomCallbacksSecondPassTask
   }
 
   void RunInternal() override {
+    TRACE_EVENT0("v8", "V8.GCPhantomHandleProcessingCallback");
     isolate()->heap()->CallGCPrologueCallbacks(
         GCType::kGCTypeProcessWeakCallbacks, kNoGCCallbackFlags);
     InvokeSecondPassPhantomCallbacks(&pending_phantom_callbacks_, isolate());