[heap] Fix cctest/test/heap tests that require ManualGCScope.

This fixed IncrementalMarkingStepMakesBigProgressWithLargeObjects
and MapRetaining.

Change-Id: I3a91684fa1f0635e94dc1883b105ce8d3381a7d2
Reviewed-on: https://chromium-review.googlesource.com/628416
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47548}

test/cctest/heap/test-heap.cc

@@ -3550,6 +3550,7 @@ TEST(DeferredHandles) {
 
 TEST(IncrementalMarkingStepMakesBigProgressWithLargeObjects) {
   if (!FLAG_incremental_marking) return;
+  ManualGCScope manual_gc_scope;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
   CompileRun("function f(n) {"
@@ -4825,7 +4826,7 @@ void CheckMapRetainingFor(int n) {
 
 TEST(MapRetaining) {
   if (!FLAG_incremental_marking) return;
-  FLAG_stress_incremental_marking = false;
+  ManualGCScope manual_gc_scope;
   CcTest::InitializeVM();
   v8::HandleScope scope(CcTest::isolate());
   CheckMapRetainingFor(FLAG_retain_maps_for_n_gc);