MSan: mark any memory allocated from the JS heap as uninitialized.

src/heap/spaces.cc

Index: src/heap/spaces.cc
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index 9be53e03f284018c8b8cafa6462f82712992fba4..6a0de0fe019ed7226108feee21d7b3f69722cabe 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -2875,6 +2875,8 @@ AllocationResult LargeObjectSpace::AllocateRaw(int object_size,
 
   HeapObject* object = page->GetObject();
 
+  MSAN_ALLOCATED_UNINITIALIZED_MEMORY(object->address(), object_size);
+
   if (Heap::ShouldZapGarbage()) {
     // Make the object consistent so the heap can be verified in OldSpaceStep.
     // We only need to do this in debug builds or if verify_heap is on.