Change download_shell_binary.py to take in path to find_depot_tools
Change download_shell_binary.py to take in path to find_depot_tools

download_shell_binary.py assumes the presence of find_depot_tools.py in a fixed
relative location, which is problematic as part of a consumable SDK. This CL
changes download_shell_binary.py to instead take in the path to
find_depot_tools.

The Chromium-side change to adjust to this update is here:
https://codereview.chromium.org/838383003/.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/841743002

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