Add timestamp to --trace-gc output.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 7e6807b0a9d3a28538a21a57839419fbb286d7db..0e59903454a043844f36045cacdd514b7f03d6f7 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1030,6 +1030,10 @@ class Isolate {
     context_exit_happened_ = context_exit_happened;
   }
 
+  double time_millis_since_init() {
+    return OS::TimeCurrentMillis() - time_millis_at_init_;
+  }
+
  private:
   Isolate();
 
@@ -1200,6 +1204,9 @@ class Isolate {
   // 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_;