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: https://github.com/nodejs/node/issues/6272

PR-URL: https://github.com/nodejs/node/pull/6669
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

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

@@ -364,7 +364,7 @@ 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();
   }
@@ -396,7 +396,7 @@ 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

@@ -155,7 +155,7 @@ Page* Page::Initialize(Heap* heap, MemoryChunk* chunk, Executability executable,
 
 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;
 }
 

deps/v8/src/heap/spaces.cc

@@ -2953,7 +2953,7 @@ 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

@@ -278,9 +278,9 @@ class MemoryChunk {
   // Only works for addresses in pointer spaces, not data or code spaces.
   static inline MemoryChunk* FromAnyPointerAddress(Heap* heap, Address addr);
 
-  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); }
 
   MemoryChunk* next_chunk() const {
     return reinterpret_cast<MemoryChunk*>(base::Acquire_Load(&next_chunk_));

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

@@ -314,7 +314,7 @@ 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++;
 
@@ -325,7 +325,7 @@ 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;
@@ -336,7 +336,7 @@ TEST(MemoryAllocator) {
   CHECK(total_pages == page_count);
 
   Page* second_page = first_page->next_page();
-  CHECK(second_page->is_valid());
+  CHECK(Page::IsValid(second_page));
   memory_allocator->Free(first_page);
   memory_allocator->Free(second_page);
   memory_allocator->TearDown();