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

src/gpu/GrResourceCache.cpp

Index: src/gpu/GrResourceCache.cpp
===================================================================
--- src/gpu/GrResourceCache.cpp	(revision 10200)
+++ src/gpu/GrResourceCache.cpp	(working copy)
@@ -164,7 +164,7 @@
 class GrTFindUnreffedFunctor {
 public:
     bool operator()(const GrResourceEntry* entry) const {
-        return 1 == entry->resource()->getRefCnt();
+        return entry->resource()->unique();
     }
 };
 
@@ -338,7 +338,7 @@
             }
 
             GrResourceEntry* prev = iter.prev();
-            if (1 == entry->fResource->getRefCnt()) {
+            if (entry->fResource->unique()) {
                 changed = true;
                 this->deleteResource(entry);
             }