This downloads the mips tool chain into tools/mips_toolchain via
runhooks if the flag download_mips_toolchain is specified for
gclient.

The currently uploaded version is 2015/1/7.

Bug: chromium:632390
Change-Id: Ied396d1a980db962773a96e9dadd3f282c7541c0
Reviewed-on: https://chromium-review.googlesource.com/892864Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50999}