Fix invalid cast in CXFA_FFNotify::OnLayoutItemRemoving().
Fix invalid cast in CXFA_FFNotify::OnLayoutItemRemoving().

There's a re-interpretation followed by a pointer-adjusting static cast
required here, not just a blatant re-interpretation.

BUG=pdfium:271
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1431313005 .

DrAlexx/pdfium,andoma/pdfium,andoma/pdfium,andoma/pdfium,andoma/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium