Make GrCacheable implement its own ref counting.

R=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/392333008
diff --git a/src/gpu/GrResourceCache.cpp b/src/gpu/GrResourceCache.cpp
index 529c3a5..ac10ebc 100644
--- a/src/gpu/GrResourceCache.cpp
+++ b/src/gpu/GrResourceCache.cpp
@@ -373,7 +373,7 @@
 }
 
 void GrResourceCache::deleteResource(GrResourceCacheEntry* entry) {
-    SkASSERT(1 == entry->fResource->getRefCnt());
+    SkASSERT(entry->fResource->unique());
 
     // remove from our cache
     fCache.remove(entry->key(), entry);