compare_build_artifacts: make ninja path command line option.

ninja seems not be in PATH when compare_build_artifacts.py is executed.
Then compare_build_artifacts cannot compare object files difference,
and just fails with exception.
To fix that, let me make ninja path given with command line option.

Bug: 862917
Change-Id: Ieb7d7bd1db30217222f5cd5006e120244da28fd5
Reviewed-on: https://chromium-review.googlesource.com/1134710
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574540}
1 file changed