Add lighter weight lock guards.

include/private/SkMutex.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkMutex_DEFINED
 #define SkMutex_DEFINED
 
@@ skipping 87 matching lines @@
     /** Assert that we're holding the mutex. */
     void assertHeld() {
         SkASSERT(fMutex);
         fMutex->assertHeld();
     }
 
 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