Implement IJS_Runtime::Destroy().

And call it from FPDF_DestroyLibrary(). Otherwise further attempts to
FPDF_InitLibraryWithConfig() can hit fail an assertion in
IJS_Runtime::Initialize().

BUG=604587

Review-Url: https://codereview.chromium.org/2103443004
diff --git a/fpdfsdk/javascript/JS_Runtime_Stub.cpp b/fpdfsdk/javascript/JS_Runtime_Stub.cpp
index 4930900..39c191a 100644
--- a/fpdfsdk/javascript/JS_Runtime_Stub.cpp
+++ b/fpdfsdk/javascript/JS_Runtime_Stub.cpp
@@ -161,6 +161,9 @@
 void IJS_Runtime::Initialize(unsigned int slot, void* isolate) {}
 
 // static
+void IJS_Runtime::Destroy() {}
+
+// static
 IJS_Runtime* IJS_Runtime::Create(CPDFDoc_Environment* pEnv) {
   return new CJS_RuntimeStub;
 }