fix up disabled bench: PDFBigDocBench

Enable with:  `extra_cflags = [ "-DSK_PDF_ENABLE_SLOW_TESTS" ]`

Change-Id: I42efbb0d90bb07538caa80da7d10d604e2297402
Reviewed-on: https://skia-review.googlesource.com/c/182761
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>