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It turned out that gclient_paths.py seems not working as expected
on canary build (could be a conflict with Google-internal src).

Implements the similar (but simplified) logic without depending on
unknown factors.

Bug: 839389
Change-Id: I70032d60098c018f139bdceb78ea73996393ddfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2029405Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736850}