Enable incremental sweeping for XFA

Although PDFium's use of STL containers prevents incremental marking,
the sweeping phase of garbage collection can be done incrementally.

REQUIRES V8 roll past 8ba4bce before landing.

Bug: chromium:1156170
Change-Id: I9f80b351b97025e0106c5c8b0c56d9ec630f5077
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/76970
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Hui Yingst <nigi@chromium.org>

fxjs/gc/heap.cpp

diff --git a/fxjs/gc/heap.cpp b/fxjs/gc/heap.cpp
index 45e3df7..7e486ec 100644
--- a/fxjs/gc/heap.cpp
+++ b/fxjs/gc/heap.cpp
@@ -77,9 +77,7 @@
           {},
           cppgc::Heap::StackSupport::kNoConservativeStackScan,
           cppgc::Heap::MarkingType::kAtomic,
-          // TODO(tsepez): switch to incremental sweeping when the issue
-          // in https://crbug.com/1156170 is resolved.
-          cppgc::Heap::SweepingType::kAtomic,
+          cppgc::Heap::SweepingType::kIncrementalAndConcurrent,
           {}});
   return FXGCScopedHeap(heap.release());
 }