[NaCl SDK] Add explicit path to gn exectuable
[NaCl SDK] Add explicit path to gn exectuable

Relying on the 'gn' wrapper in depot_tools doesn't work
as its currently broken on the official builders.

CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_nacl_sdk;tryserver.chromium.mac:mac_nacl_sdk;tryserver.chromium.win:win_nacl_sdk
BUG=588794

Review URL: https://codereview.chromium.org/1718283002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#376830}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk