Mark pdfium chromium_code.
Mark pdfium chromium_code.

Now that Chromium's build/common.gypi no longer does weird things
with -Wsign-compare, this should work.

BUG=pdfium:29
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1557923004 .

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