tools/gn: convert NULL to nullptr all over it.

nullptr is one of the C++11 allowed features. See https://chromium-cpp.appspot.com/ and https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/4mijeJHzxLg.

This patch was generated with the following command lines:

$ ninja -C out/Debug -t compdb cxx > out/Debug/compile_commands.json
$ cd out/Debug
$ find ../../tools/gn -name '*.cc'  | xargs -n 16 -P 32 \
~/src/repos/llvm/build/bin/clang-modernize -use-nullptr -p . -include \
tools/gn -format -style=Chromium
$ ninja gn gn_unittests

BUG=None
TEST=compiles + gn_unittests
[email protected]

Review URL: https://codereview.chromium.org/798333005

Cr-Commit-Position: refs/heads/master@{#309505}
diff --git a/tools/gn/filesystem_utils.cc b/tools/gn/filesystem_utils.cc
index 700b7b8..89e8a30 100644
--- a/tools/gn/filesystem_utils.cc
+++ b/tools/gn/filesystem_utils.cc
@@ -422,7 +422,7 @@
 }
 
 void NormalizePath(std::string* path) {
-  char* pathbuf = path->empty() ? NULL : &(*path)[0];
+  char* pathbuf = path->empty() ? nullptr : &(*path)[0];
 
   // top_index is the first character we can modify in the path. Anything
   // before this indicates where the path is relative to.