Enable incremental marking when --expose-gc is turned on.

src/extensions/gc-extension.cc

Index: src/extensions/gc-extension.cc
diff --git a/src/extensions/gc-extension.cc b/src/extensions/gc-extension.cc
index 1b5fb5d8c47768e00d85ccf8b20f5da95decc817..c79024939250f975e481e378d5eaa7fb43f8548d 100644
--- a/src/extensions/gc-extension.cc
+++ b/src/extensions/gc-extension.cc
@@ -45,8 +45,8 @@ void GCExtension::GC(const v8::FunctionCallbackInfo<v8::Value>& args) {
     isolate->heap()->CollectGarbage(
         NEW_SPACE, "gc extension", v8::kGCCallbackFlagForced);
   } else {
-    isolate->heap()->CollectAllGarbage(
-        Heap::kNoGCFlags, "gc extension", v8::kGCCallbackFlagForced);
+    isolate->heap()->CollectAllGarbage(Heap::kAbortIncrementalMarkingMask,
+        "gc extension", v8::kGCCallbackFlagForced);
   }
 }