Update link to chromium src.
Update link to chromium src.

This CL updates the documentation link to the Chromium PDF plugin to
point to Gitiles instead of the SVN repo.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1407313002 .

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