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

src/ports/SkThread_win.cpp

Index: src/ports/SkThread_win.cpp
===================================================================
--- src/ports/SkThread_win.cpp	(revision 10052)
+++ src/ports/SkThread_win.cpp	(working copy)
@@ -49,6 +49,10 @@
 }
 void sk_membar_aquire__after_atomic_conditional_inc() { }
 
+int32_t sk_atomic_unprotected_read(const volatile int32_t & x) {
+    return x;
+}
+
 SkMutex::SkMutex() {
     SK_COMPILE_ASSERT(sizeof(fStorage) > sizeof(CRITICAL_SECTION),
                       NotEnoughSizeForCriticalSection);