Switch CPU profile start/stop markers to monotonic time.

src/profile-generator.cc

Index: src/profile-generator.cc
diff --git a/src/profile-generator.cc b/src/profile-generator.cc
index 6bd446e0c49d471348fc2548beb342b9e1804ab1..ae37648e5ea99cb1cf754cbafb56c5f020575ce1 100644
--- a/src/profile-generator.cc
+++ b/src/profile-generator.cc
@@ -355,8 +355,7 @@ void ProfileTree::TraverseDepthFirst(Callback* callback) {
 CpuProfile::CpuProfile(const char* title, bool record_samples)
     : title_(title),
       record_samples_(record_samples),
-      start_time_(Time::NowFromSystemTime()) {
-  timer_.Start();
+      start_time_(TimeTicks::HighResolutionNow()) {
 }
 
 
@@ -367,7 +366,7 @@ void CpuProfile::AddPath(const Vector<CodeEntry*>& path) {
 
 
 void CpuProfile::CalculateTotalTicksAndSamplingRate() {
-  end_time_ = start_time_ + timer_.Elapsed();
+  end_time_ = TimeTicks::HighResolutionNow();
 }