deps: backport IsValid changes from 4e8736d in V8

V8 erroneously did null pointer checks on `this`.
It can lead to a SIGSEGV crash if node is compiled with GCC 6.
Backport relevant changes from [1] that fix this issue.

[1]: https://codereview.chromium.org/1900423002

Fixes: nodejs#6272
PR-URL: nodejs#6544
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>

deps/v8/src/heap/incremental-marking.cc

@@ -404,7 +404,7 @@ void IncrementalMarking::DeactivateIncrementalWriteBarrier() {
   DeactivateIncrementalWriteBarrierForSpace(heap_->new_space());
 
   LargePage* lop = heap_->lo_space()->first_page();
-  while (lop->is_valid()) {
+  while (LargePage::IsValid(lop)) {
     SetOldSpacePageFlags(lop, false, false);
     lop = lop->next_page();
   }
@@ -436,7 +436,7 @@ void IncrementalMarking::ActivateIncrementalWriteBarrier() {
   ActivateIncrementalWriteBarrier(heap_->new_space());
 
   LargePage* lop = heap_->lo_space()->first_page();
-  while (lop->is_valid()) {
+  while (LargePage::IsValid(lop)) {
     SetOldSpacePageFlags(lop, true, is_compacting_);
     lop = lop->next_page();
   }

deps/v8/src/heap/spaces-inl.h

@@ -284,14 +284,14 @@ void MemoryChunk::IncrementLiveBytesFromMutator(HeapObject* object, int by) {
 
 bool PagedSpace::Contains(Address addr) {
   Page* p = Page::FromAddress(addr);
-  if (!p->is_valid()) return false;
+  if (!Page::IsValid(p)) return false;
   return p->owner() == this;
 }
 
 bool PagedSpace::Contains(Object* o) {
   if (!o->IsHeapObject()) return false;
   Page* p = Page::FromAddress(HeapObject::cast(o)->address());
-  if (!p->is_valid()) return false;
+  if (!Page::IsValid(p)) return false;
   return p->owner() == this;
 }
 

deps/v8/src/heap/spaces.cc

@@ -3044,7 +3044,7 @@ LargePage* LargeObjectSpace::FindPage(Address a) {
   if (e != NULL) {
     DCHECK(e->value != NULL);
     LargePage* page = reinterpret_cast<LargePage*>(e->value);
-    DCHECK(page->is_valid());
+    DCHECK(LargePage::IsValid(page));
     if (page->Contains(a)) {
       return page;
     }

deps/v8/src/heap/spaces.h

@@ -454,9 +454,9 @@ class MemoryChunk {
              !chunk->high_water_mark_.TrySetValue(old_mark, new_mark));
   }
 
-  Address address() { return reinterpret_cast<Address>(this); }
+  static bool IsValid(MemoryChunk* chunk) { return chunk != nullptr; }
 
-  bool is_valid() { return address() != NULL; }
+  Address address() { return reinterpret_cast<Address>(this); }
 
   base::Mutex* mutex() { return mutex_; }
 

deps/v8/test/cctest/heap/test-spaces.cc

@@ -321,7 +321,7 @@ TEST(MemoryAllocator) {
         faked_space.AreaSize(), &faked_space, NOT_EXECUTABLE);
 
     first_page->InsertAfter(faked_space.anchor()->prev_page());
-    CHECK(first_page->is_valid());
+    CHECK(Page::IsValid(first_page));
     CHECK(first_page->next_page() == faked_space.anchor());
     total_pages++;
 
@@ -332,7 +332,7 @@ TEST(MemoryAllocator) {
     // Again, we should get n or n - 1 pages.
     Page* other = memory_allocator->AllocatePage(faked_space.AreaSize(),
                                                  &faked_space, NOT_EXECUTABLE);
-    CHECK(other->is_valid());
+    CHECK(Page::IsValid(other));
     total_pages++;
     other->InsertAfter(first_page);
     int page_count = 0;
@@ -343,7 +343,7 @@ TEST(MemoryAllocator) {
     CHECK(total_pages == page_count);
 
     Page* second_page = first_page->next_page();
-    CHECK(second_page->is_valid());
+    CHECK(Page::IsValid(second_page));
 
     // OldSpace's destructor will tear down the space and free up all pages.
   }