Add clear() to SizedArray

Change-Id: I237c344378ecf4966e12fb2aee6975696909e178
diff --git a/aos/containers/sized_array.h b/aos/containers/sized_array.h
index 6d4209a..34e9206 100644
--- a/aos/containers/sized_array.h
+++ b/aos/containers/sized_array.h
@@ -109,6 +109,8 @@
     --size_;
   }
 
+  void clear() { size_ = 0; }
+
   // These allow access to the underlying storage. The data here may be outside
   // the current logical extents of the container.
   const array &backing_array() const { return array_; }