Add lighter weight lock guards.

src/core/SkSharedMutex.h

Index: src/core/SkSharedMutex.h
diff --git a/src/core/SkSharedMutex.h b/src/core/SkSharedMutex.h
index ea948d3f6c90bdf03cce8be77c609b32f541f5c5..840c2d36adf152b50087675eb1140c78001dd407 100644
--- a/src/core/SkSharedMutex.h
+++ b/src/core/SkSharedMutex.h
@@ -69,4 +69,14 @@ 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