Add timestamp to --trace-gc output.

src/isolate.h

 // 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 1012 matching lines @@
     thread_local_top_.top_lookup_result_ = top;
   }
 
   bool context_exit_happened() {
     return context_exit_happened_;
   }
   void set_context_exit_happened(bool context_exit_happened) {
     context_exit_happened_ = context_exit_happened;
   }
 
+  double time_millis_since_init() {
+    return OS::TimeCurrentMillis() - time_millis_at_init_;
+  }
+
  private:
   Isolate();
 
   // The per-process lock should be acquired before the ThreadDataTable is
   // modified.
   class ThreadDataTable {
    public:
     ThreadDataTable();
     ~ThreadDataTable();
 
@@ skipping 150 matching lines @@
   unibrow::Mapping<unibrow::Ecma262Canonicalize>
       regexp_macro_assembler_canonicalize_;
   RegExpStack* regexp_stack_;
   unibrow::Mapping<unibrow::Ecma262Canonicalize> interp_canonicalize_mapping_;
   void* embedder_data_;
 
   // The garbage collector should be a little more aggressive when it knows
   // that a context was recently exited.
   bool context_exit_happened_;
 
+  // Time stamp at initialization.
+  double time_millis_at_init_;
+
 #if defined(V8_TARGET_ARCH_ARM) && !defined(__arm__) || \
     defined(V8_TARGET_ARCH_MIPS) && !defined(__mips__)
   bool simulator_initialized_;
   HashMap* simulator_i_cache_;
   Redirection* simulator_redirection_;
 #endif
 
 #ifdef DEBUG
   // A static array of histogram info for each type.
   HistogramInfo heap_histograms_[LAST_TYPE + 1];
@@ skipping 178 matching lines @@
 
 // Mark the global context with out of memory.
 inline void Context::mark_out_of_memory() {
   global_context()->set_out_of_memory(HEAP->true_value());
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_ISOLATE_H_