[Minor Cleanup] Remove unnecessary constructor from WebCompositionUnderline.h

This constructor has no references to it. Hence, removing it.

BUG=None

Review URL: https://codereview.chromium.org/426573004

git-svn-id: svn://svn.chromium.org/blink/trunk@179083 bbb929c8-8fbe-4397-9dbb-9b2b20218538