[offthread] Avoid invalid live byte update for off-thread page

During off-thread space merge, we free the linear allocation area in the
off-thread space. Since the off-thread space isn't marked, we have to
make sure that we don't try to compensate for black allocated live bytes.

Bug: chromium:1011762
Change-Id: Id2eb2212dc25e78952f817482abcdb4b49f3a373
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2111224Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66823}

src/heap/spaces.cc

@@ -1120,7 +1120,7 @@ size_t Page::ShrinkToHighWaterMark() {
 void Page::CreateBlackArea(Address start, Address end) {
   DCHECK(heap()->incremental_marking()->black_allocation());
   DCHECK_EQ(Page::FromAddress(start), this);
-  DCHECK_NE(start, end);
+  DCHECK_LT(start, end);
   DCHECK_EQ(Page::FromAddress(end - 1), this);
   IncrementalMarking::MarkingState* marking_state =
       heap()->incremental_marking()->marking_state();
@@ -1132,7 +1132,7 @@ void Page::CreateBlackArea(Address start, Address end) {
 void Page::DestroyBlackArea(Address start, Address end) {
   DCHECK(heap()->incremental_marking()->black_allocation());
   DCHECK_EQ(Page::FromAddress(start), this);
-  DCHECK_NE(start, end);
+  DCHECK_LT(start, end);
   DCHECK_EQ(Page::FromAddress(end - 1), this);
   IncrementalMarking::MarkingState* marking_state =
       heap()->incremental_marking()->marking_state();
@@ -1998,7 +1998,8 @@ void PagedSpace::FreeLinearAllocationArea() {
     return;
   }
 
-  if (heap()->incremental_marking()->black_allocation()) {
+  if (!is_off_thread_space() &&
+      heap()->incremental_marking()->black_allocation()) {
     Page* page = Page::FromAllocationAreaAddress(current_top);
 
     // Clear the bits in the unused black area.