Created an ArraySize utility function.

[ Merge of http://go/wvgerrit/82232 ]

The new function `ArraySize` will only work for compile-time fixed-length
arrays, and will provide compiler errors if the provided argument is not an
array.

This will replace the commonly used macro `N_ELEM()` which uses `sizeof()`.

Test: Linux unittest
Bug: 137041745
Change-Id: I0f5c268197a8062a99ccba43c73349e97f66eb02

libwvdrmengine/cdm/util/include/arraysize.h

@@ -0,0 +1,20 @@
+// Copyright 2019 Google LLC. All Rights Reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine Master
+// License Agreement.
+
+#ifndef WVCDM_UTIL_ARRAYSIZE_H_
+#define WVCDM_UTIL_ARRAYSIZE_H_
+
+#include <stdint.h>
+
+namespace wvcdm {
+
+// Returns the size of a fixed-length array.
+template <typename T, size_t N>
+constexpr size_t ArraySize(const T (&)[N]) {
+  return N;
+}
+
+}  // namespace wvcdm
+
+#endif  // WVCDM_UTIL_ARRAYSIZE_H_