Fix race condition in parallel font initialization.

Uses a mutex to guard construction of the singleton, which initialies
the non-threadsafe libfontconfig.  Without this change, the 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, tomhudson@google.com

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

Author: tomhudson@chromium.org

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