Merge to XFA: Don't dispose the isolate if its provided by the embedder

(cherry picked from commit 7ea305f1bd4efc1838fba053e4399358d6765ec7)
Original Review URL: https://codereview.chromium.org/1269783005 .

BUG=chromium:515718
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1270293002 .
diff --git a/fpdfsdk/include/javascript/JS_Runtime.h b/fpdfsdk/include/javascript/JS_Runtime.h
index 1fb0348..1e9f3a9 100644
--- a/fpdfsdk/include/javascript/JS_Runtime.h
+++ b/fpdfsdk/include/javascript/JS_Runtime.h
@@ -73,6 +73,7 @@
   CJS_FieldEvent* m_pFieldEventPath;
 
   v8::Isolate* m_isolate;
+  bool m_isolateManaged;
   nonstd::unique_ptr<CJS_ArrayBufferAllocator> m_pArrayBufferAllocator;
   v8::Global<v8::Context> m_context;
 };