Fix leak in FontService PPAPI fallback function

Replace manual memory management of FontConfig options with unique_ptr's
and address a container-overflow issue in the unit test.

Bug: 961018
Change-Id: Ief6a44abb98a4d48372440bc62cb801bfddabeb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604263
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658486}
2 files changed