GC: Add tracing event for rescanning large objects on newspace evacuation

BUG=

Review URL: https://codereview.chromium.org/1269753002

Cr-Commit-Position: refs/heads/master@{#29945}

src/heap/gc-tracer.cc

@@ -426,6 +426,8 @@ void GCTracer::PrintNVP() const {
       PrintF("sweepcode=%.2f ", current_.scopes[Scope::MC_SWEEP_CODE]);
       PrintF("sweepcell=%.2f ", current_.scopes[Scope::MC_SWEEP_CELL]);
       PrintF("sweepmap=%.2f ", current_.scopes[Scope::MC_SWEEP_MAP]);
+      PrintF("rescan_lo=%.2f ",
+             current_.scopes[Scope::MC_RESCAN_LARGE_OBJECTS]);
       PrintF("evacuate=%.1f ", current_.scopes[Scope::MC_EVACUATE_PAGES]);
       PrintF("new_new=%.1f ",
              current_.scopes[Scope::MC_UPDATE_NEW_TO_NEW_POINTERS]);

src/heap/gc-tracer.h

@@ -104,6 +104,7 @@ class GCTracer {
       MC_SWEEP_CODE,
       MC_SWEEP_CELL,
       MC_SWEEP_MAP,
+      MC_RESCAN_LARGE_OBJECTS,
       MC_EVACUATE_PAGES,
       MC_UPDATE_NEW_TO_NEW_POINTERS,
       MC_UPDATE_ROOT_TO_NEW_POINTERS,

src/heap/mark-compact.cc

@@ -3695,6 +3695,8 @@ void MarkCompactCollector::EvacuateNewSpaceAndCandidates() {
     }
 
     if (compacting_ && was_marked_incrementally_) {
+      GCTracer::Scope gc_scope(heap()->tracer(),
+                               GCTracer::Scope::MC_RESCAN_LARGE_OBJECTS);
       // It's difficult to filter out slots recorded for large objects.
       LargeObjectIterator it(heap_->lo_space());
       for (HeapObject* obj = it.Next(); obj != NULL; obj = it.Next()) {