Do not invoke V8 CPPCG after heap destroyed in tests

This is a bug in the pdfium_test harness where a V8 cppgc message
loop may be pumped after the heap is destroyed (by the scoped document
going out of scope at the end of ProcessPdf(). We've already idled the
loop to emptyness at line 1136, so there's no need after return.

Bug: chromium:1325664
Change-Id: I34d5a82551789d6359dbb7a58185bb9ff83d6629
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/93730
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/samples/pdfium_test.cc b/samples/pdfium_test.cc
index 2c76808..5703d83 100644
--- a/samples/pdfium_test.cc
+++ b/samples/pdfium_test.cc
@@ -1378,7 +1378,6 @@
 
     ProcessPdf(filename, file_contents.get(), file_length, options, events,
                idler);
-    idler();
 
 #ifdef ENABLE_CALLGRIND
     if (options.callgrind_delimiters)