Segv when PDF-side JS object property getter invoked from XFA.

The PDF-side native objects require that the current v8 context
has been set-up to point at the state (via CJS_Runtime) for the
getters, setters, and methods to operate against. XFA doesn't
supply a context with that state, so at the first opportunity for
a PDF-side object to be leaked to XFA, set up the context to mimic
the PDF side.

Changed FXJS_GetRuntimeFromIsolate() to FXJS_GetRuntimeFromV8Context()
for consistency with the newly added method.

BUG=pdfium:266
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/1412103010 .
diff --git a/fpdfsdk/include/jsapi/fxjs_v8.h b/fpdfsdk/include/jsapi/fxjs_v8.h
index 7051840..001823f 100644
--- a/fpdfsdk/include/jsapi/fxjs_v8.h
+++ b/fpdfsdk/include/jsapi/fxjs_v8.h
@@ -133,7 +133,13 @@
                             v8::Global<v8::Context>& v8PersistentContext);
 void FXJS_ReleaseRuntime(v8::Isolate* pIsolate,
                          v8::Global<v8::Context>& v8PersistentContext);
-IJS_Runtime* FXJS_GetRuntimeFromIsolate(v8::Isolate* pIsolate);
+
+// Called as part of FXJS_InitializeRuntime, exposed so PDF can make its
+// own contexts compatible with XFA or vice versa.
+void FXJS_SetRuntimeForV8Context(v8::Local<v8::Context> v8Context,
+                                 IJS_Runtime* pIRuntime);
+
+IJS_Runtime* FXJS_GetRuntimeFromV8Context(v8::Local<v8::Context> v8Context);
 
 // Called after FXJS_InitializeRuntime call made.
 int FXJS_Execute(v8::Isolate* pIsolate,