[pdfium] Set up cppgc before V8 isolate

To allow V8 to use cppgc, cppgc has to be initialized before the
initialization of the V8 isolate. This CL changes the initialization
order in pdfium accordingly.

Bug: 42203693
Change-Id: Icb781c372bc3006dd2572621a36873b3b54224ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6276532
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1421296}

pdf/pdfium/pdfium_engine.cc

@@ -275,6 +275,9 @@ bool IsV8Initialized() {
 }
 
 void SetUpV8() {
+#if defined(PDF_ENABLE_XFA)
+  gin::InitializeCppgcFromV8Platform();
+#endif
   if (!gin::IsolateHolder::Initialized()) {
     // TODO(crbug.com/40142415): V8 flags for the PDF Viewer need to be set up
     // as soon as the renderer process is created in the constructor of
@@ -292,10 +295,6 @@ void SetUpV8() {
       new gin::IsolateHolder(base::SingleThreadTaskRunner::GetCurrentDefault(),
                              gin::IsolateHolder::kSingleThread,
                              gin::IsolateHolder::IsolateType::kUtility);
-
-#if defined(PDF_ENABLE_XFA)
-  gin::InitializeCppgcFromV8Platform();
-#endif
 }
 
 void TearDownV8() {