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

Only (unused) API removed.
TBR=reed@google.com

BUG=skia:3160

Review URL: https://codereview.chromium.org/752263002
diff --git a/tests/RefCntTest.cpp b/tests/RefCntTest.cpp
index cf6ca3e..cf4acc6 100644
--- a/tests/RefCntTest.cpp
+++ b/tests/RefCntTest.cpp
@@ -49,7 +49,7 @@
     thing1.join();
     thing2.join();
 
-    REPORTER_ASSERT(reporter, ref->getRefCnt() == 1);
+    REPORTER_ASSERT(reporter, ref->unique());
     ref->unref();
 }
 
@@ -93,7 +93,7 @@
     thing3.join();
     thing4.join();
 
-    REPORTER_ASSERT(reporter, ref->getRefCnt() == 1);
+    REPORTER_ASSERT(reporter, ref->unique());
     REPORTER_ASSERT(reporter, ref->getWeakCnt() == 1);
     ref->unref();
 }