Use methods from depot_tool's Changelist object instead of Rietveld RPC

This will make support for both Gerrit and Rietveld easier.

BUG=skia:5676
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2307593002
NOTRY=true

Review-Url: https://codereview.chromium.org/2307593002
1 file changed