Switch CPU profile start/stop markers to monotonic time.

include/v8-profiler.h

Index: include/v8-profiler.h
diff --git a/include/v8-profiler.h b/include/v8-profiler.h
index f5b760a402538e7b2c6cffe3a5ffb06e9fd55805..026715fda785463d8c743313576ac8bfcadce29d 100644
--- a/include/v8-profiler.h
+++ b/include/v8-profiler.h
@@ -118,14 +118,15 @@ class V8_EXPORT CpuProfile {
   const CpuProfileNode* GetSample(int index) const;
 
   /**
-    * Returns time when the profile recording started (in microseconds
-    * since the Epoch).
+    * Returns time when the profile recording was started (in microseconds)
+    * since some unspecified starting point.
     */
   int64_t GetStartTime() const;
 
   /**
-    * Returns time when the profile recording was stopped (in microseconds
-    * since the Epoch).
+    * Returns time when the profile recording was stopped (in microseconds)
+    * since some unspecified starting point. The point is however equal to the
+    * starting point used by GetStartTime.
     */
   int64_t GetEndTime() const;