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/tests/Matrix44Test.cpp b/tests/Matrix44Test.cpp
index 254a41c..28f7d76 100644
--- a/tests/Matrix44Test.cpp
+++ b/tests/Matrix44Test.cpp
@@ -94,7 +94,7 @@
     }
 
     // Verify that kIdentity_Constructor really does initialize to an identity matrix.
-    testMatrix = 0;
+    testMatrix = nullptr;
     testMatrix = new(placeholderMatrix) SkMatrix44(SkMatrix44::kIdentity_Constructor);
     REPORTER_ASSERT(reporter, testMatrix == placeholderMatrix);
     REPORTER_ASSERT(reporter, testMatrix->isIdentity());