Add debug mode to shared mutex.

Review URL: https://codereview.chromium.org/1307863009
diff --git a/include/core/SkTypes.h b/include/core/SkTypes.h
index a5468f3..d9254ac 100644
--- a/include/core/SkTypes.h
+++ b/include/core/SkTypes.h
@@ -106,6 +106,7 @@
 #ifdef SK_DEBUG
     #define SkASSERT(cond)              SK_ALWAYSBREAK(cond)
     #define SkDEBUGFAIL(message)        SkASSERT(false && message)
+    #define SkDEBUGFAILF(fmt, ...)      SkASSERTF(false, fmt, ##__VA_ARGS__)
     #define SkDEBUGCODE(code)           code
     #define SkDECLAREPARAM(type, var)   , type var
     #define SkPARAM(var)                , var