MFH: r352849

Fix build when depot_tools is not present locally (which is usually the
case on non-development machines).

PR: ports/189230
Submitted by: Jia-Shiun Li <jiashiun@gmail.com>
Obtained from: https://codereview.chromium.org/171513010/
Tested by: gjb

Approved by: portmgr (erwin)