Prefix "depot_tools" to PATH instead of appending. This will ensure that
"depot_tools" pulls resources from the recipe checkout as a preference,
not as a last resort.

Use "env_prefixes" instead of "env" to prefix PATH.

Remove Git speed limit settings, which are taken care of by the Git
wrapper now.

BUG=chromium:738176
TEST=expectations
R=hinoka@chromium.org, iannucci@chromium.org

Change-Id: I2e509a331266e867be323bc70e3bc0f08f85246f
Reviewed-on: https://chromium-review.googlesource.com/556287Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>