Create libsampler as V8 sampler library.

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index b5db5fe4295d11d17589fdeb07fefdbf2cde7e81..43154927ea0af585e449e79a8c6cc7684d281e75 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -6331,6 +6331,12 @@ class V8_EXPORT Isolate {
    */
   void VisitWeakHandles(PersistentHandleVisitor* visitor);
 
+  /**
+   * Check if this isolate is in use.
+   * True if at least one thread Enter'ed this isolate.
+   */
+  bool IsInUse();

[alph, 2016/05/24 22:19:21]

Can GetStackSample just return an empty stack if the isolate is not entered?
I.e. do we really need to expose this in the API?

[lpy, 2016/05/24 23:13:10]

I think we can. But I added this API in order to stick the logic in signal
handler since I can't verify the correctness without it.

+
  private:
   template <class K, class V, class Traits>
   friend class PersistentValueMapBase;