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>
19 files changed