Remove partially implemented PDF FFI_* handlers
Remove partially implemented PDF FFI_* handlers

Returning NULL will allow pdfium to continue, with some
missing content, rather than crashing outright.

BUG=pdfium:382
BUG=pdfium:383

Review URL: https://codereview.chromium.org/1709833002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#376210}

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