Use nullptr instead of 0 in call to FontConfig.

Review-Url: https://codereview.chromium.org/2000253005
diff --git a/src/ports/SkFontConfigInterface_direct.cpp b/src/ports/SkFontConfigInterface_direct.cpp
index d6fa96a..ccea794 100644
--- a/src/ports/SkFontConfigInterface_direct.cpp
+++ b/src/ports/SkFontConfigInterface_direct.cpp
@@ -632,7 +632,7 @@
     }
 
     FcResult result;
-    FcFontSet* font_set = FcFontSort(0, pattern, 0, 0, &result);
+    FcFontSet* font_set = FcFontSort(nullptr, pattern, 0, nullptr, &result);
     if (!font_set) {
         FcPatternDestroy(pattern);
         return false;