under third_party/* directory.

e.g., FindThirdPartyDeps() currently returns
".../third_party/cld_3/src/src" where it should be
".../third_party/cld_3".

I can reproduce the issue by:
$ git cl patch 2797403002
$ tools/licenses.py --gn-target=//ios/web_view \
  --gn-out-dir=out/Debug-iphonesimulator license_file

I'm planning to reuse this script to generate LICENSE file for
//ios/web_view, and encountered this issue.

Review-Url: https://codereview.chromium.org/2802723004
Cr-Commit-Position: refs/heads/master@{#465497}