Add a constexpr constructor for TVariable

BUG=angleproject:2267
TEST=angle_unittests

Change-Id: I9b7e01801caa7235ac5e2d4212ea92e38c1f774d
Reviewed-on: https://chromium-review.googlesource.com/908752
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/common/angleutils.h b/src/common/angleutils.h
index db95173..829897c 100644
--- a/src/common/angleutils.h
+++ b/src/common/angleutils.h
@@ -30,7 +30,7 @@
 class NonCopyable
 {
   protected:
-    NonCopyable() = default;
+    constexpr NonCopyable() = default;
     ~NonCopyable() = default;
 
   private: