Don't use UTF16 length as length for MUTF8.

Bug 11367555.

Change-Id: Ia0b07072a1a49d435c3b71ed9a668b316b7ff5d8
diff --git a/compiler/driver/compiler_driver.h b/compiler/driver/compiler_driver.h
index 971021f..fe21eff 100644
--- a/compiler/driver/compiler_driver.h
+++ b/compiler/driver/compiler_driver.h
@@ -309,7 +309,7 @@
   }
 
   // Checks if class specified by type_idx is one of the image_classes_
-  bool IsImageClass(const StringPiece& descriptor) const;
+  bool IsImageClass(const char* descriptor) const;
 
   void RecordClassStatus(ClassReference ref, mirror::Class::Status status)
       LOCKS_EXCLUDED(compiled_classes_lock_);