libchromeos: Replace NULL with nullptr

BUG=None
TEST=FEATURES=test emerge-link libchromeos

Change-Id: I41c74e364a8299e148db3e81e5146755cc3e45ea
Reviewed-on: https://chromium-review.googlesource.com/214833
Reviewed-by: Ben Chan <[email protected]>
Tested-by: Alex Vakulenko <[email protected]>
Commit-Queue: Alex Vakulenko <[email protected]>
13 files changed