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When the pdf_enable_xfa GYP_DEFINES flag is present, pass
-DPDF_ENABLE_XFA during the build of pdf engine. If the
XFA branch is present, the public XFA header will notice this
flag, and define PDF_USE_XFA in response, which the engine
tests to control its usage of XFA features.  When chrome is
built against master, PDF_USE_XFA is never defined, so this
still works regardless of the setting of the GYP define.

Review URL: https://codereview.chromium.org/1511443004

Cr-Commit-Position: refs/heads/master@{#364466}