make SkRefCnt::getRefCnt() debug-only, remove it from SkNVRefCnt.

tests/SurfaceTest.cpp

Index: tests/SurfaceTest.cpp
diff --git a/tests/SurfaceTest.cpp b/tests/SurfaceTest.cpp
index 1913fcb1467cb6d53e7f73d16f55fe3688927a91..6422669055ad2bbd632f41def7dce01d930493cf 100644
--- a/tests/SurfaceTest.cpp
+++ b/tests/SurfaceTest.cpp
@@ -77,11 +77,11 @@ static void test_image(skiatest::Reporter* reporter) {
     size_t size = info.getSafeSize(rowBytes);
     SkData* data = SkData::NewUninitialized(size);
 
-    REPORTER_ASSERT(reporter, 1 == data->getRefCnt());
+    REPORTER_ASSERT(reporter, data->unique());
     SkImage* image = SkImage::NewRasterData(info, data, rowBytes);
-    REPORTER_ASSERT(reporter, 2 == data->getRefCnt());
+    REPORTER_ASSERT(reporter, !data->unique());
     image->unref();
-    REPORTER_ASSERT(reporter, 1 == data->getRefCnt());
+    REPORTER_ASSERT(reporter, data->unique());
     data->unref();
 }