Fix build failure on Win64 since r14458.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 5c1cde2bf526297371769d68390398021faa0746..dc7de2a4dea5b43ddd4a54e3ff49bf75c0752a34 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -2480,7 +2480,7 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_SuspendJSGeneratorObject) {
   // allocation fails.
   intptr_t offset = frame->pc() - function->code()->instruction_start();
   ASSERT(offset > 0 && Smi::IsValid(offset));
-  generator_object->set_continuation(offset);
+  generator_object->set_continuation(static_cast<int>(offset));
 
   // It's possible for the context to be other than the initial context even if
   // there is no stack handler active.  For example, this is the case in the