Add reset() and position() to CodedOutputByteBufferNano.

Change-Id: Ie2a9e36276ac35e10b3f8d379b5742d50a0374e9

javanano/src/main/java/com/google/protobuf/nano/CodedOutputByteBufferNano.java

@@ -898,6 +898,23 @@ public final class CodedOutputByteBufferNano {
     }
   }
 
+  /**
+   * Returns the position within the internal buffer.
+   */
+  public int position() {
+    return buffer.position();
+  }
+
+  /**
+   * Resets the position within the internal buffer to zero.
+   *
+   * @see #position
+   * @see #spaceLeft
+   */
+  public void reset() {
+    buffer.clear();
+  }
+
   /**
    * If you create a CodedOutputStream around a simple flat array, you must
    * not attempt to write more bytes than the array has space.  Otherwise,