Introduce concurrent on-stack replacement.

src/deoptimizer.h

Index: src/deoptimizer.h
diff --git a/src/deoptimizer.h b/src/deoptimizer.h
index 0d62bd09de386dabf9c3a026805f19a5cf3e3c89..7ee1ae12c6ed578a5368088a47e8e9d2454c78fd 100644
--- a/src/deoptimizer.h
+++ b/src/deoptimizer.h
@@ -262,7 +262,7 @@ class Deoptimizer : public Malloced {
                                                     Address pc_after);
 
   // Verify that all back edges of a certain loop depth are patched.
-  static void VerifyInterruptCode(Isolate* isolate,
+  static bool VerifyInterruptCode(Isolate* isolate,
                                   Code* unoptimized_code,
                                   int loop_nesting_level);
 #endif  // DEBUG