Check for existence of depot_tools
Check for existence of depot_tools

BUG=skia:

Review-Url: https://codereview.chromium.org/2197493004

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot