Add third_party/sfntly/src to .gitignore.
Add third_party/sfntly/src to .gitignore.

While file location has been changed in crrev.com/1367323002, .gitignore
is not updated correctly.

BUG=none
TBR=thestig@chromium.org

Review URL: https://codereview.chromium.org/1377803002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#351280}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk