[api] GetHeapStatistics: Only report backing stores as external memory

Whether backing stores are on V8's heap or not is opaque to JavaScript.
This is not true for other external memory such as DOM which is why only
backingstores are considered as on heap.

Bug: chromium:914304
Change-Id: Idfeb9815daeee239a6a5b95077421e555bfe5fda
Reviewed-on: https://chromium-review.googlesource.com/c/1454726Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59389}

src/api.cc

@@ -8405,7 +8405,7 @@ void Isolate::GetHeapStatistics(HeapStatistics* heap_statistics) {
   heap_statistics->malloced_memory_ =
       isolate->allocator()->GetCurrentMemoryUsage() +
       isolate->wasm_engine()->allocator()->GetCurrentMemoryUsage();
-  heap_statistics->external_memory_ = isolate->heap()->external_memory();
+  heap_statistics->external_memory_ = isolate->heap()->backing_store_bytes();
   heap_statistics->peak_malloced_memory_ =
       isolate->allocator()->GetMaxMemoryUsage() +
       isolate->wasm_engine()->allocator()->GetMaxMemoryUsage();