Introduce DisallowDeoptimization scope.

src/assert-scope.h

Index: src/assert-scope.h
diff --git a/src/assert-scope.h b/src/assert-scope.h
index 428e6d007ef8d5a9a5d03ba9484b402a16674bec..2bb4623e72742d360d668136502b9ba7c03fc2b6 100644
--- a/src/assert-scope.h
+++ b/src/assert-scope.h
@@ -50,7 +50,8 @@ enum PerThreadAssertType {
 enum PerIsolateAssertType {
   JAVASCRIPT_EXECUTION_ASSERT,
   JAVASCRIPT_EXECUTION_THROWS,
-  ALLOCATION_FAILURE_ASSERT
+  ALLOCATION_FAILURE_ASSERT,
+  DEOPTIMIZATION_ASSERT
 };
 
 
@@ -268,6 +269,14 @@ typedef PerIsolateAssertScopeDebugOnly<ALLOCATION_FAILURE_ASSERT, false>
 typedef PerIsolateAssertScopeDebugOnly<ALLOCATION_FAILURE_ASSERT, true>
     AllowAllocationFailure;
 
+// Scope to document where we do not expect deoptimization.
+typedef PerIsolateAssertScopeDebugOnly<DEOPTIMIZATION_ASSERT, false>
+    DisallowDeoptimization;
+
+// Scope to introduce an exception to DisallowDeoptimization.
+typedef PerIsolateAssertScopeDebugOnly<DEOPTIMIZATION_ASSERT, true>
+    AllowDeoptimization;
+
 } }  // namespace v8::internal
 
 #endif  // V8_ASSERT_SCOPE_H_