Enable ClangTidy check modernize-use-nullptr.

https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

The check converts the usage of null pointer constants (eg. NULL, 0) to
use the new C++11 nullptr keyword.

Change-Id: Iaea2d843154c70e49d62affdc5dceb3bca8c1089
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310297
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/.clang-tidy b/.clang-tidy
index 7c4b363..cb38913 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -9,6 +9,7 @@
     llvm-namespace-comment,
     misc-definitions-in-headers,
     modernize-make-unique,
+    modernize-use-nullptr,
     modernize-use-override,
     performance-for-range-copy,
     performance-unnecessary-copy-initialization,