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>
diff --git a/metrics/timer.cc b/metrics/timer.cc
index dd78425..99f68fe 100644
--- a/metrics/timer.cc
+++ b/metrics/timer.cc
@@ -86,7 +86,7 @@
 }
 
 // static
-MetricsLibraryInterface* TimerReporter::metrics_lib_ = NULL;
+MetricsLibraryInterface* TimerReporter::metrics_lib_ = nullptr;
 
 TimerReporter::TimerReporter(const std::string& histogram_name, int min,
                              int max, int num_buckets)