Add --src-dir flag to landmines.py.
Add --src-dir flag to landmines.py.

This will make it possible to reuse the script in projects
with different directory layouts than Chromium, since the
current version assumes the root is two levels above the
script location.

BUG=webrtc:5077
TESTED=Manually running the script and running runhooks.

Review URL: https://codereview.chromium.org/1407733002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354083}

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