[heap] Guard against re-entering GC on external memory change.

TBR=hpayer@chromium.org
BUG=chromium:729868,chromium:729521
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

Review-Url: https://codereview.chromium.org/2929463002
Cr-Commit-Position: refs/heads/master@{#45745}

src/api.cc

@@ -8150,6 +8150,7 @@ void Isolate::ReportExternalAllocationLimitReached() {
 
 void Isolate::CheckMemoryPressure() {
   i::Heap* heap = reinterpret_cast<i::Isolate*>(this)->heap();
+  if (heap->gc_state() != i::Heap::NOT_IN_GC) return;
   heap->CheckMemoryPressure();
 }