Fix crash in V8 during serializing objects requiring alignment.

Review URL: https://codereview.chromium.org/793753002

Cr-Commit-Position: refs/heads/master@{#25764}

src/serialize.cc

@@ -1657,14 +1657,17 @@ void Serializer::ObjectSerializer::SerializePrologue(AllocationSpace space,
     }
     back_reference = serializer_->AllocateLargeObject(size);
   } else {
+    bool needs_double_align = false;
     if (object_->NeedsToEnsureDoubleAlignment()) {
       // Add wriggle room for double alignment padding.
       back_reference = serializer_->Allocate(space, size + kPointerSize);
-      sink_->PutInt(kDoubleAlignmentSentinel, "DoubleAlignSentinel");
+      needs_double_align = true;
     } else {
       back_reference = serializer_->Allocate(space, size);
     }
     sink_->Put(kNewObject + reference_representation_ + space, "NewObject");
+    if (needs_double_align)
+      sink_->PutInt(kDoubleAlignmentSentinel, "DoubleAlignSentinel");
     int encoded_size = size >> kObjectAlignmentBits;
     DCHECK_NE(kDoubleAlignmentSentinel, encoded_size);
     sink_->PutInt(encoded_size, "ObjectSizeInWords");