Add timestamp to --trace-gc output.

src/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 6577 matching lines @@
   if (FLAG_print_cumulative_gc_stat) {
     heap_->max_gc_pause_ = Max(heap_->max_gc_pause_, time);
     heap_->max_alive_after_gc_ = Max(heap_->max_alive_after_gc_,
                                      heap_->alive_after_last_gc_);
     if (!first_gc) {
       heap_->min_in_mutator_ = Min(heap_->min_in_mutator_,
                                    static_cast<int>(spent_in_mutator_));
     }
   }
 
+  PrintF("%8.0f ms: ", heap_->isolate()->time_millis_since_init());
+
   if (!FLAG_trace_gc_nvp) {
     int external_time = static_cast<int>(scopes_[Scope::EXTERNAL]);
 
     double end_memory_size_mb =
         static_cast<double>(heap_->isolate()->memory_allocator()->Size()) / MB;
 
     PrintF("%s %.1f (%.1f) -> %.1f (%.1f) MB, ",
            CollectorString(),
            static_cast<double>(start_object_size_) / MB,
            static_cast<double>(start_memory_size_) / MB,
@@ skipping 270 matching lines @@
   isolate_->heap()->store_buffer()->Compact();
   isolate_->heap()->store_buffer()->Filter(MemoryChunk::ABOUT_TO_BE_FREED);
   for (chunk = chunks_queued_for_free_; chunk != NULL; chunk = next) {
     next = chunk->next_chunk();
     isolate_->memory_allocator()->Free(chunk);
   }
   chunks_queued_for_free_ = NULL;
 }
 
 } }  // namespace v8::internal