Make app.alert() actually print its message under pdfium_test.
Make app.alert() actually print its message under pdfium_test.

This is needed as an aid to testing.

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/788433002

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