Fix an "unsigned int"/"size_t" discrepancy.

Found by the Mac build.

Change-Id: I4111e95c80b7ea812a915255c06ec47e6caa7d48
diff --git a/src/assembler_arm.h b/src/assembler_arm.h
index f6be2b9..2a721ea 100644
--- a/src/assembler_arm.h
+++ b/src/assembler_arm.h
@@ -481,8 +481,7 @@
   virtual void CopyRef(FrameOffset dest, FrameOffset src,
                        ManagedRegister scratch);
 
-  virtual void Copy(FrameOffset dest, FrameOffset src, ManagedRegister scratch,
-                    unsigned int size);
+  virtual void Copy(FrameOffset dest, FrameOffset src, ManagedRegister scratch, size_t size);
 
   virtual void Copy(FrameOffset dest, ManagedRegister src_base, Offset src_offset,
                     ManagedRegister scratch, size_t size);