[heap] Activate memory reducer on external memory activity.

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 6094e30d2fa8100eaf7af09f17576f761b111594..f2564f9f3eb83cfb1042b2d583b1295c03e88332 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -2831,6 +2831,9 @@ bool Isolate::Init(Deserializer* des) {
            Internals::kExternalMemoryOffset);
   CHECK_EQ(static_cast<int>(OFFSET_OF(Isolate, heap_.external_memory_limit_)),
            Internals::kExternalMemoryLimitOffset);
+  CHECK_EQ(static_cast<int>(
+               OFFSET_OF(Isolate, heap_.external_memory_at_last_mark_compact_)),
+           Internals::kExternalMemoryAtLastMarkCompactOffset);
 
   time_millis_at_init_ = heap_.MonotonicallyIncreasingTimeInMs();