Add a 'unique' method to SkRefCnt, document the usage, and add support.

src/ports/SkThread_none.cpp

Index: src/ports/SkThread_none.cpp
===================================================================
--- src/ports/SkThread_none.cpp	(revision 10052)
+++ src/ports/SkThread_none.cpp	(working copy)
@@ -36,6 +36,10 @@
 }
 void sk_membar_aquire__after_atomic_conditional_inc() { }
 
+int32_t sk_atomic_unprotected_read(const volatile int32_t & x) {
+    return x;
+}
+
 SkMutex::SkMutex() {}
 
 SkMutex::~SkMutex() {}