Return start/end profiling time in microseconds instead of milliseconds

src/profile-generator.h

Index: src/profile-generator.h
diff --git a/src/profile-generator.h b/src/profile-generator.h
index ce2dd307e3183f9ef55082da14a590429c740586..0cc397ed9bf24583d5d12f69b078157b19176312 100644
--- a/src/profile-generator.h
+++ b/src/profile-generator.h
@@ -216,8 +216,8 @@ class CpuProfile {
   int samples_count() const { return samples_.length(); }
   ProfileNode* sample(int index) const { return samples_.at(index); }
 
-  double start_time_ms() const { return start_time_ms_; }
-  double end_time_ms() const { return end_time_ms_; }
+  int64_t start_time_us() const { return start_time_us_; }
+  int64_t end_time_us() const { return end_time_us_; }
 
   void UpdateTicksScale();
 
@@ -228,8 +228,8 @@ class CpuProfile {
   const char* title_;
   unsigned uid_;
   bool record_samples_;
-  double start_time_ms_;
-  double end_time_ms_;
+  int64_t start_time_us_;
+  int64_t end_time_us_;
   List<ProfileNode*> samples_;
   ProfileTree top_down_;