Always adjust amount of external memory when change is reported.

test/cctest/test-api.cc

Index: test/cctest/test-api.cc
diff --git a/test/cctest/test-api.cc b/test/cctest/test-api.cc
index 4d39aa543d6f79e96d1e5879f94d53638e1eebc4..b8cfe4f09749ac39b979f2a80e741a463490661c 100644
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -15199,6 +15199,12 @@ THREADED_TEST(ExternalAllocatedMemory) {
            isolate->AdjustAmountOfExternalAllocatedMemory(kSize));
   CHECK_EQ(baseline,
            isolate->AdjustAmountOfExternalAllocatedMemory(-kSize));
+  const int64_t kTriggerGCSize =
+      v8::internal::Internals::kExternalAllocationLimit + 1;
+  CHECK_EQ(baseline + kTriggerGCSize,
+           isolate->AdjustAmountOfExternalAllocatedMemory(kTriggerGCSize));
+  CHECK_EQ(baseline,
+           isolate->AdjustAmountOfExternalAllocatedMemory(-kTriggerGCSize));
 }