Add lighter weight lock guards.

src/core/SkSharedMutex.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 SkSharedLock_DEFINED
 #define SkSharedLock_DEFINED
 
@@ skipping 51 matching lines @@
     SkSemaphore       fSharedQueue;
     SkSemaphore       fExclusiveQueue;
 #endif  // SK_DEBUG
 };
 
 #ifndef SK_DEBUG
 inline void SkSharedMutex::assertHeld() const {};
 inline void SkSharedMutex::assertHeldShared() const {};
 #endif  // SK_DEBUG
 
+class SkAutoSharedMutexShared {

[bungeman-skia, 2015/10/21 21:19:09]

After some thinking about it I do get the name, but it doesn't seem that
obvious. On the other hand, I'm not sure I have a better suggestion.

+public:
+    SkAutoSharedMutexShared(SkSharedMutex& lock) : fLock(lock) { lock.acquireShared(); }
+    ~SkAutoSharedMutexShared() { fLock.releaseShared(); }
+private:
+    SkSharedMutex& fLock;
+};
+
+#define SkAutoSharedMutexShared(...) SK_REQUIRE_LOCAL_VAR(SkAutoSharedMutexShared)
+
 #endif // SkSharedLock_DEFINED