Remove a few V8 includes from header files.

Change-Id: I087c4e712aa87f99ada4ef515f5b361037a5dae0
Reviewed-on: https://pdfium-review.googlesource.com/c/44257
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/testing/embedder_test.cpp b/testing/embedder_test.cpp
index 10b944d..6dd13e2 100644
--- a/testing/embedder_test.cpp
+++ b/testing/embedder_test.cpp
@@ -94,6 +94,12 @@
   delete loader_;
 }
 
+#ifdef PDF_ENABLE_V8
+void EmbedderTest::SetExternalIsolate(void* isolate) {
+  external_isolate_ = static_cast<v8::Isolate*>(isolate);
+}
+#endif  // PDF_ENABLE_V8
+
 bool EmbedderTest::CreateEmptyDocument() {
   document_ = FPDF_CreateNewDocument();
   if (!document_)