build/vs_toolchain.py requires find_depot_tools.py, but it's not
available to a project that gets build/ via DEPS and does not also
get all of tools/ via DEPS.  The native_client repo needs to use
build/vs_toolchain.py, but it does not copy chromium/src/tools/ and
it should not have to duplicate find_depot_tools.py just to use
build/vs_toolchain.py.

The native_client repo also needs a standalone script to print out
the depot_tools directory location, and find_depot_tools.py can
serve that purpose too with some trivial additions.

BUG= 512869
R=ben@chromium.org, dbeam@chromium.org, dpranke@chromium.org, jbudorick@chromium.org, klundberg@chromium.org, phajdan.jr@chromium.org, sky@chromium.org, tbreisacher@chromium.org

Review URL: https://codereview.chromium.org/1418513007

Cr-Commit-Position: refs/heads/master@{#356141}