update_engine: Fix lint issues in delta_performer.cc

This is necessary to allow repo upload succeed in the next cherry-picked
patch.

Bug: none
Test: FEATURES="test" emerge-amd64-generic update_engine

Change-Id: Ibf922e4fec6dc62b477588d0748ecaed60867b63
Reviewed-on: https://chromium-review.googlesource.com/641957


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
(cherry picked from commit 69583ba5)