Enable XFA-Forms for the PDFium plugin.

This is a change to gather stability, performance, and size
data and will be rolled back by 2016-03-15.

This is the gyp portion of the change; the GN variables
controlling the same things currently reside in the pdfium
product itsef.

Review URL: https://codereview.chromium.org/1761673002

Cr-Commit-Position: refs/heads/master@{#379622}
1 file changed