XFA: Rename freetype to fx_freetype in pdfium.gyp.
XFA: Rename freetype to fx_freetype in pdfium.gyp.

Forgot to do so in commit 461b634b8b45bf022412b364fac37f6ad3cafb69.

TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1431073003 .

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