This revision works around a suspected bug in gcc which was causing
build failures. The compiler would complain that ApplyScrollOffset was
protected, and therefore inaccessible, if we capture the return value
of that function within BoundsWithOffset. I don't think this actually
violates any C++ rules, and other compilers agree. To work around the
problem, this revision wraps the call to ApplyScrollOffset in an
immediately-invoked lambda, which satisfies gcc for some reason.
Differential Revision: https://phabricator.services.mozilla.com/D174697
65baef59a3