Expose functionality to create a stack trace to internal vm code.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 35284)
+++ runtime/vm/object.h	(working copy)
@@ -6497,8 +6497,11 @@
                             Heap::Space space = Heap::kNew);
 
   RawString* FullStacktrace() const;
-  const char* ToCStringInternal(intptr_t* frame_index) const;
 
+  // The argument 'max_frames' limits the number of printed frames.
+  const char* ToCStringInternal(intptr_t* frame_index,
+                                intptr_t max_frames = kMaxInt32) const;
+
  private:
   void set_code_array(const Array& code_array) const;
   void set_pc_offset_array(const Array& pc_offset_array) const;