Add lighter weight lock guards.

include/private/SkMutex.h

Index: include/private/SkMutex.h
diff --git a/include/private/SkMutex.h b/include/private/SkMutex.h
index c3e9919e52a98548d957ef10a727e1fd812aafb8..da44f648d0b17595b21ad32830e5cb52b8b46048 100644
--- a/include/private/SkMutex.h
+++ b/include/private/SkMutex.h
@@ -105,7 +105,21 @@ private:
     Lock* fMutex;
 };
 
+// SkAutoTExclusive is a lighter weight version of SkAutoTAcquire. It assumes that there is a valid
+// mutex, thus removing the check for the null pointer.
+template <typename Lock>
+class SkAutoTExclusive {
+public:
+    SkAutoTExclusive(Lock& lock) : fLock(lock) { lock.acquire(); }
+    ~SkAutoTExclusive() { fLock.release(); }
+private:
+    Lock &fLock;
+};
+
 typedef SkAutoTAcquire<SkBaseMutex> SkAutoMutexAcquire;
 #define SkAutoMutexAcquire(...) SK_REQUIRE_LOCAL_VAR(SkAutoMutexAcquire)
 
+typedef SkAutoTExclusive<SkMutex> SkAutoMutexExclusive;
+#define SkAutoMutexExclusive(...) SK_REQUIRE_LOCAL_VAR(SkAutoMutexExclusive)
+
 #endif//SkMutex_DEFINED