[heap] User safer root set accessor when possible.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index f19e52156d83ab8538bf7283ae5b01f2a00acabe..ddf06449d809f5b6048eb2c7a63ce659bb0cd145 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -3373,7 +3373,7 @@ bool Heap::RootCanBeWrittenAfterInitialization(Heap::RootListIndex root_index) {
 
 bool Heap::RootCanBeTreatedAsConstant(RootListIndex root_index) {
   return !RootCanBeWrittenAfterInitialization(root_index) &&
-         !InNewSpace(roots_array_start()[root_index]);
+         !InNewSpace(root(root_index));
 }