Uses optional mutex to guard construction of the singleton, which
initializes the non-threadsafe libfontconfig. Without this change,
Skia's parallel path ops test runner crashes 6/10 and hangs 2/10 on
startup; with this change, 0/10 problems.

BUG=skia:2693
R=mtklein@google.com, bungeman@google.com, reed@google.com

Committed: https://skia.googlesource.com/skia/+/df022f5972ae6a2a1d96d15c50eca52cade3abd8

Committed: https://skia.googlesource.com/skia/+/60b08a0adfe73f593af62c8d3f55958438360e1b

Author: tomhudson@chromium.org

Review URL: https://codereview.chromium.org/355573006
4 files changed