Add support for changing roots through the root visitor callback.

Needed for copying collectors.

Change-Id: Icc4a342a57e0cfb79587edb02ef8c85e08808877
diff --git a/runtime/mirror/string.cc b/runtime/mirror/string.cc
index 7d968c7..b82683e 100644
--- a/runtime/mirror/string.cc
+++ b/runtime/mirror/string.cc
@@ -33,6 +33,10 @@
   return GetFieldObject<const CharArray*>(ValueOffset(), false);
 }
 
+CharArray* String::GetCharArray() {
+  return GetFieldObject<CharArray*>(ValueOffset(), false);
+}
+
 void String::ComputeHashCode() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
   SetHashCode(ComputeUtf16Hash(GetCharArray(), GetOffset(), GetLength()));
 }