Support profiling based on linux kernel performance events.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index e9e7c2236749b188c1ff2fb15dac2ec53fc15763..e29d3b234212e2d7d1837f0cd59c0d334276695c 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -257,11 +257,16 @@ class OS {
   static char* StrChr(char* str, int c);
   static void StrNCpy(Vector<char> dest, const char* src, size_t n);
 
-  // Support for profiler.  Can do nothing, in which case ticks
-  // occuring in shared libraries will not be properly accounted
-  // for.
+  // Support for the profiler.  Can do nothing, in which case ticks
+  // occuring in shared libraries will not be properly accounted for.
   static void LogSharedLibraryAddresses();
 
+  // Support for the profiler.  Notifies the external profiling
+  // process that a code moving garbage collection starts.  Can do
+  // nothing, in which case the code objects must not move (e.g., by
+  // using --never-compact) if accurate profiling is desired.
+  static void SignalCodeMovingGC();
+
   // The return value indicates the CPU features we are sure of because of the
   // OS.  For example MacOSX doesn't run on any x86 CPUs that don't have SSE2
   // instructions.