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

src/gpu/GrEffect.cpp

Index: src/gpu/GrEffect.cpp
===================================================================
--- src/gpu/GrEffect.cpp	(revision 10200)
+++ src/gpu/GrEffect.cpp	(working copy)
@@ -63,7 +63,7 @@
 SK_DEFINE_INST_COUNT(GrEffectRef)
 
 GrEffectRef::~GrEffectRef() {
-    GrAssert(1 == this->getRefCnt());
+    GrAssert(this->unique());
     fEffect->EffectRefDestroyed();
     fEffect->unref();
 }