Fix leak of GrResourceCacheEntry

R=robertphillips@google.com
TBR=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/502543002
diff --git a/src/gpu/GrResourceCache.h b/src/gpu/GrResourceCache.h
index 235b11a..82ffbea 100644
--- a/src/gpu/GrResourceCache.h
+++ b/src/gpu/GrResourceCache.h
@@ -152,6 +152,7 @@
     SK_DECLARE_INTERNAL_LLIST_INTERFACE(GrResourceCacheEntry);
 
     friend class GrResourceCache;
+    friend class GrContext;
 };
 
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