Make GrGpuResource::gpuMemorySize non-virtual w/ onGpuMemorySize virtual impl

BUG=skia:2889

Review URL: https://codereview.chromium.org/702413003
diff --git a/tests/ResourceCacheTest.cpp b/tests/ResourceCacheTest.cpp
index fb17113..7cebddb 100644
--- a/tests/ResourceCacheTest.cpp
+++ b/tests/ResourceCacheTest.cpp
@@ -99,8 +99,6 @@
         this->didChangeGpuMemorySize();
     }
 
-    size_t gpuMemorySize() const SK_OVERRIDE { return fSize; }
-
     static int NumAlive() { return fNumAlive; }
 
     void setDeleteWhenDestroyed(GrResourceCache* cache, TestResource* resource) {
@@ -109,6 +107,8 @@
     }
 
 private:
+    size_t onGpuMemorySize() const SK_OVERRIDE { return fSize; }
+
     GrResourceCache* fCache;
     TestResource* fToDelete;
     size_t fSize;