Document RootIndexMap's requirement for immovable roots.

R=mstarzinger@chromium.org

Change-Id: I27cf279c77204becc449221e1bcebeacbc275792
Reviewed-on: https://chromium-review.googlesource.com/565281
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46517}

src/address-map.cc

@@ -20,6 +20,8 @@ RootIndexMap::RootIndexMap(Isolate* isolate) {
     if (!root->IsHeapObject()) continue;
     // Omit root entries that can be written after initialization. They must
     // not be referenced through the root list in the snapshot.
+    // Since we map the raw address of an root item to its root list index, the
+    // raw address must be constant, i.e. the object must be immovable.
     if (isolate->heap()->RootCanBeTreatedAsConstant(root_index)) {
       HeapObject* heap_object = HeapObject::cast(root);
       Maybe<uint32_t> maybe_index = map_->Get(heap_object);

src/heap/heap.cc

@@ -2945,8 +2945,10 @@ bool Heap::RootCanBeWrittenAfterInitialization(Heap::RootListIndex root_index) {
 }
 
 bool Heap::RootCanBeTreatedAsConstant(RootListIndex root_index) {
-  return !RootCanBeWrittenAfterInitialization(root_index) &&
+  bool can_be = !RootCanBeWrittenAfterInitialization(root_index) &&
                 !InNewSpace(root(root_index));
+  DCHECK_IMPLIES(can_be, IsImmovable(HeapObject::cast(root(root_index))));
+  return can_be;
 }
 
 int Heap::FullSizeNumberStringCacheLength() {