Add timestamps to CPU profile samples.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 43856f6596d7fc3a7767305d710a2962dadba671..694a51f69c9915ceae543b6af3fa5b6e82cc1a51 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -7067,6 +7067,12 @@ const CpuProfileNode* CpuProfile::GetSample(int index) const {
 }
 
 
+int64_t CpuProfile::GetSampleTimestamp(int index) const {
+  const i::CpuProfile* profile = reinterpret_cast<const i::CpuProfile*>(this);
+  return (profile->sample_timestamp(index) - i::TimeTicks()).InMicroseconds();
+}
+
+
 int64_t CpuProfile::GetStartTime() const {
   const i::CpuProfile* profile = reinterpret_cast<const i::CpuProfile*>(this);
   return (profile->start_time() - i::TimeTicks()).InMicroseconds();