platform2: Replace NULL with nullptr in minor components

Replace NULL with nullptr in C++11-enabled components of platform2
that don't have too many occurrences of NULL. Other major
"offenders" will be handled separately.

BUG=None
TEST=FEATURES=test emerge-link app-shell-launcher buffet easy-unlock platform2 metrics wimax_manager

Change-Id: I61b25a057e3d6865908bc74f9f3d4cb55e08af26
Reviewed-on: https://chromium-review.googlesource.com/214837
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>

buffet/main.cc

diff --git a/buffet/main.cc b/buffet/main.cc
index fde5c86..0f2c6d0 100644
--- a/buffet/main.cc
+++ b/buffet/main.cc
@@ -61,7 +61,7 @@
   const auto log_path =
       base::StringPrintf("%s/buffet.%s",
                          logs_dir.c_str(),
-                         GetTimeAsString(::time(NULL)).c_str());
+                         GetTimeAsString(::time(nullptr)).c_str());
   mkdir(logs_dir.c_str(), 0755);
   SetupLogSymlink(log_symlink, log_path);
   return log_symlink;