Add Isolate::DiscardThreadSpecificMetadata method to embedder API.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 132edbfd5546242d6a39e85e149a486ea0003ccb..e42cc0270d1e26a81a35bf26541efb549e5d6abc 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -7162,6 +7162,12 @@ void Isolate::Dispose() {
 }
 
 
+void Isolate::DiscardThreadSpecificMetadata() {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  isolate->DiscardPerThreadDataForThisThread();
+}
+
+
 void Isolate::Enter() {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
   isolate->Enter();