Fix thread unsafe mutex initialization.

BUG=skia:2779
R=robertphillips@google.com, mtklein@google.com, reed@android.com, bsalomon@google.com

Author: bungeman@google.com

Review URL: https://codereview.chromium.org/419113002
diff --git a/src/ports/SkFontHost_mac.cpp b/src/ports/SkFontHost_mac.cpp
index a8e1b57..2d985f9 100755
--- a/src/ports/SkFontHost_mac.cpp
+++ b/src/ports/SkFontHost_mac.cpp
@@ -526,9 +526,9 @@
     return ctFont ? NewFromFontRef(ctFont, familyName, false) : NULL;
 }
 
+SK_DECLARE_STATIC_MUTEX(gGetDefaultFaceMutex);
 static SkTypeface* GetDefaultFace() {
-    SK_DECLARE_STATIC_MUTEX(gMutex);
-    SkAutoMutexAcquire ma(gMutex);
+    SkAutoMutexAcquire ma(gGetDefaultFaceMutex);
 
     static SkTypeface* gDefaultFace;