Instrumented libraries: Remove dependence on GYP_DEFINES

This CL ports download_binaries.py and unpack_binaries.py from using
GYP_DEFINES to gn args.  Previously, there was a gotcha step where you
had to run 'gclient sync' with the right GYP_DEFINES to actually download
the prebuilt instrumented libraries.  This CL removes that step.

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_msan_rel_ng;master.tryserver.chromium.linux:linux_chromium_chromeos_msan_rel_ng
R=eugenis@chromium.org
BUG=705072

Review-Url: https://codereview.chromium.org/2775913002
Cr-Commit-Position: refs/heads/master@{#459630}
4 files changed