[heap] Fix test-api/Regress528 for concurrent marking.

BUG=chromium:694255
TBR=mlippautz@chromium.org

Change-Id: I5c6fbb392d68a348e3a8b3fb52368232883e9931
Reviewed-on: https://chromium-review.googlesource.com/535594Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45940}

test/cctest/test-api.cc

@@ -18921,6 +18921,7 @@ THREADED_TEST(SpaghettiStackReThrow) {
 
 
 TEST(Regress528) {
+  ManualGCScope manual_gc_scope;
   v8::V8::Initialize();
   v8::Isolate* isolate = CcTest::isolate();
   i::FLAG_retain_maps_for_n_gc = 0;