Use subprocess2 for downloading tools.
Use subprocess2 for downloading tools.

Change to use exactly the same approach as when gclient hooks
are executing [1]. It should be safe since it's using the DEPS-pinned
depot_tools in third_party. Hopefully this solves the race condition
problems we've been seeing in crbug.com/773671

[1]: https://cs.chromium.org/chromium/tools/depot_tools/gclient.py?rcl=b3ce73d028b1d44137d533220fd41be31bc31801&l=214

Bug: chromium:773671
Change-Id: Ia003dbca394e42556afa1a416fcb4844b960ad6c
No-try: True
Reviewed-on: https://webrtc-review.googlesource.com/8820
Reviewed-by: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Commit-Queue: Henrik Kjellander <60c11d3e646624f25a02b88f51d51ca6b7ff7c72@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#20256}
TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc