[api] Fix AdjustAmountOfExternalAllocatedMemory memory reducer

The reduer should only fire on increasing memory.

R=ulan@chromium.org

Change-Id: I4abd956ea14730b223724a01af819be574b1aa3a
Reviewed-on: https://chromium-review.googlesource.com/c/1308354Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57149}

include/v8.h

@@ -10406,7 +10406,9 @@ int64_t Isolate::AdjustAmountOfExternalAllocatedMemory(
   *external_memory = amount;
 
   int64_t allocation_diff_since_last_mc =
-      *external_memory_at_last_mc - *external_memory;
+      *external_memory - *external_memory_at_last_mc;
+  // Only check memory pressure and potentially trigger GC if the amount of
+  // external memory increased.
   if (allocation_diff_since_last_mc > kMemoryReducerActivationLimit) {
     CheckMemoryPressure();
   }