Make GrResourceCache use a priority queue of purgeable resources.

src/gpu/GrGpuResourceCacheAccess.h

Index: src/gpu/GrGpuResourceCacheAccess.h
diff --git a/src/gpu/GrGpuResourceCacheAccess.h b/src/gpu/GrGpuResourceCacheAccess.h
index 922e3b36b5a47541874507dcba23bc008c4f5ab1..52294cef90e99fa56bd0613f610510f727ffdb56 100644
--- a/src/gpu/GrGpuResourceCacheAccess.h
+++ b/src/gpu/GrGpuResourceCacheAccess.h
@@ -55,6 +55,11 @@ private:
         }
     }
 
+    uint32_t timestamp() const { return fResource->fTimestamp; }
+    void setTimestamp(uint32_t ts) { fResource->fTimestamp = ts; }
+
+    int* accessCacheIndex() const { return &fResource->fCacheArrayIndex; }
+
     CacheAccess(GrGpuResource* resource) : fResource(resource) {}
     CacheAccess(const CacheAccess& that) : fResource(that.fResource) {}
     CacheAccess& operator=(const CacheAccess&); // unimpl