Add support for changing roots through the root visitor callback.

Needed for copying collectors.

Change-Id: Icc4a342a57e0cfb79587edb02ef8c85e08808877
diff --git a/runtime/mirror/string.h b/runtime/mirror/string.h
index bf545ea..01d8f31 100644
--- a/runtime/mirror/string.h
+++ b/runtime/mirror/string.h
@@ -44,6 +44,7 @@
   }
 
   const CharArray* GetCharArray() const;
+  CharArray* GetCharArray();
 
   int32_t GetOffset() const {
     int32_t result = GetField32(OffsetOffset(), false);