Mutex protect SkFontMgr_DirectWrite cache.

SkFontMgr_DirectWrite is constant, except for its typeface cache.
Protect this cache in the face of multiple threads.

R=robertphillips@google.com

Review URL: https://codereview.chromium.org/26023002

git-svn-id: http://skia.googlecode.com/svn/trunk@11615 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed