tools/gn: Pass clang-modernize -use-nullptr over it.

tools/gn/args.cc

Index: tools/gn/args.cc
diff --git a/tools/gn/args.cc b/tools/gn/args.cc
index a59371a73852649dde02065275b85c6da98875a7..c7c8a0939ca605ec632f6074874a507f3a6b92a1 100644
--- a/tools/gn/args.cc
+++ b/tools/gn/args.cc
@@ -98,7 +98,7 @@ const Value* Args::GetArgOverride(const char* name) const {
   Scope::KeyValueMap::const_iterator found =
       all_overrides_.find(base::StringPiece(name));
   if (found == all_overrides_.end())
-    return NULL;
+    return nullptr;
   return &found->second;
 }
 
@@ -209,7 +209,7 @@ void Args::SetSystemVarsLocked(Scope* dest) const {
   lock_.AssertAcquired();
 
   // Host OS.
-  const char* os = NULL;
+  const char* os = nullptr;
 #if defined(OS_WIN)
   os = "win";
 #elif defined(OS_MACOSX)
@@ -221,15 +221,15 @@ void Args::SetSystemVarsLocked(Scope* dest) const {
 #else
   #error Unknown OS type.
 #endif
-  Value os_val(NULL, std::string(os));
-  dest->SetValue(variables::kBuildOs, os_val, NULL);
-  dest->SetValue(variables::kOs, os_val, NULL);
+  Value os_val(nullptr, std::string(os));
+  dest->SetValue(variables::kBuildOs, os_val, nullptr);
+  dest->SetValue(variables::kOs, os_val, nullptr);
 
   // Host architecture.
   static const char kX86[] = "x86";
   static const char kX64[] = "x64";
   static const char kArm[] = "arm";
-  const char* arch = NULL;
+  const char* arch = nullptr;
 
   // Set the CPU architecture based on the underlying OS, not
   // whatever the current bit-tedness of the GN binary is.
@@ -243,9 +243,9 @@ void Args::SetSystemVarsLocked(Scope* dest) const {
   else
     CHECK(false) << "OS architecture not handled.";
 
-  Value arch_val(NULL, std::string(arch));
-  dest->SetValue(variables::kBuildCpuArch, arch_val, NULL);
-  dest->SetValue(variables::kCpuArch, arch_val, NULL);
+  Value arch_val(nullptr, std::string(arch));
+  dest->SetValue(variables::kBuildCpuArch, arch_val, nullptr);
+  dest->SetValue(variables::kCpuArch, arch_val, nullptr);
 
   // Save the OS and architecture as build arguments that are implicitly
   // declared. This is so they can be overridden in a toolchain build args