Create a cppgc::Heap for xfa fuzzers

Restore one initialization lost in
  https://pdfium-review.googlesource.com/c/pdfium/+/73670

Bug: chromium:1130100
Change-Id: I2b412184af942b85a58696250b42bd1912ef2237
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/73830
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

testing/fuzzers/xfa_process_state.cc

diff --git a/testing/fuzzers/xfa_process_state.cc b/testing/fuzzers/xfa_process_state.cc
index 0a8cf52..a95b22f 100644
--- a/testing/fuzzers/xfa_process_state.cc
+++ b/testing/fuzzers/xfa_process_state.cc
@@ -4,8 +4,10 @@
 
 #include "testing/fuzzers/xfa_process_state.h"
 
+#include "fxjs/gc/heap.h"
+
 XFAProcessState::XFAProcessState(v8::Platform* platform, v8::Isolate* isolate)
-    : platform_(platform), isolate_(isolate) {}
+    : platform_(platform), isolate_(isolate), heap_(FXGC_CreateHeap()) {}
 
 XFAProcessState::~XFAProcessState() = default;