[heap] Fix DCHECK in CodePageMemoryModificationScope to allow non-space MemoryChunks.

Bug: chromium:774108,v8:6792
Change-Id: Ia312b8c6ca8608f8ac96e8caefd061900b40079b
Reviewed-on: https://chromium-review.googlesource.com/739481Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48976}

src/heap/heap-inl.h

@@ -609,7 +609,10 @@ CodePageMemoryModificationScope::CodePageMemoryModificationScope(
       scope_active_(FLAG_write_protect_code_memory &&
                     chunk_->IsFlagSet(MemoryChunk::IS_EXECUTABLE)) {
   if (scope_active_) {
-    DCHECK(chunk_->owner()->identity() == CODE_SPACE ||
+    // TODO(hpayer): owner() can only be null if we use the MemoryChunk outside
+    // of spaces. We actually should not do that and we should untangle this.
+    DCHECK(chunk_->owner() == nullptr ||
+           chunk_->owner()->identity() == CODE_SPACE ||
            (chunk_->owner()->identity() == LO_SPACE &&
             chunk_->IsFlagSet(MemoryChunk::IS_EXECUTABLE)));
     chunk_->SetReadAndWritable();