[heap] MinorMC: pause observers before toggling gc state

Bug: v8:7407
Change-Id: Icb13921872a9e41dced3ae95ff37ddf72ae30568
Reviewed-on: https://chromium-review.googlesource.com/902009Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#51101}

src/heap/heap.cc

@@ -1816,12 +1816,12 @@ void Heap::MarkCompact() {
 void Heap::MinorMarkCompact() {
   DCHECK(FLAG_minor_mc);
 
+  PauseAllocationObserversScope pause_observers(this);
   SetGCState(MINOR_MARK_COMPACT);
   LOG(isolate_, ResourceEvent("MinorMarkCompact", "begin"));
 
   TRACE_GC(tracer(), GCTracer::Scope::MINOR_MC);
   AlwaysAllocateScope always_allocate(isolate());
-  PauseAllocationObserversScope pause_observers(this);
   IncrementalMarking::PauseBlackAllocationScope pause_black_allocation(
       incremental_marking());
   CodeSpaceMemoryModificationScope code_modifcation(this);

src/heap/spaces.cc

@@ -71,6 +71,8 @@ bool HeapObjectIterator::AdvanceToNextPage() {
 
 PauseAllocationObserversScope::PauseAllocationObserversScope(Heap* heap)
     : heap_(heap) {
+  DCHECK_EQ(heap->gc_state(), Heap::NOT_IN_GC);
+
   for (SpaceIterator it(heap_); it.has_next();) {
     it.next()->PauseAllocationObservers();
   }