[debugger] Abort incremental marking in debug::GetLoadedScripts.

The GC performed in GetLoadedScripts currently finalizes incremental
marking, which fails in some tests due to floating garbage.

BUG=chromium:694255

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ic1fdd2fb123c02ed7bea4c9fb53024574758b536
Reviewed-on: https://chromium-review.googlesource.com/533334Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45915}

src/api.cc

@@ -9614,7 +9614,7 @@ void debug::GetLoadedScripts(v8::Isolate* v8_isolate,
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
   ENTER_V8_NO_SCRIPT_NO_EXCEPTION(isolate);
   // TODO(kozyatinskiy): remove this GC once tests are dealt with.
-  isolate->heap()->CollectAllGarbage(i::Heap::kFinalizeIncrementalMarkingMask,
+  isolate->heap()->CollectAllGarbage(i::Heap::kMakeHeapIterableMask,
                                      i::GarbageCollectionReason::kDebugger);
   {
     i::DisallowHeapAllocation no_gc;