Revert of SK_DECLARE_STATIC_MUTEX -> static SkMutex

src/ports/SkFontMgr_fontconfig.cpp

Index: src/ports/SkFontMgr_fontconfig.cpp
diff --git a/src/ports/SkFontMgr_fontconfig.cpp b/src/ports/SkFontMgr_fontconfig.cpp
index 32f10a491c9540707b2aac1556fe48cfbff09759..610a3001fb2c66e5df68e2fe08f0c34d7c0644f0 100644
--- a/src/ports/SkFontMgr_fontconfig.cpp
+++ b/src/ports/SkFontMgr_fontconfig.cpp
@@ -62,7 +62,7 @@
 
 // Fontconfig is not threadsafe before 2.10.91. Before that, we lock with a global mutex.
 // See https://bug.skia.org/1497 for background.
-static SkMutex gFCMutex;
+SK_DECLARE_STATIC_MUTEX(gFCMutex);
 
 #ifdef SK_DEBUG
 void* CreateThreadFcLocked() { return new bool(false); }