What steps will reproduce the problem?
(1) Go to https://www.chromium.org/developers/testing/instrumented-libraries-for-dynamic-tools
(2) Try to follow instructions under "Using pre-built binaries"

What is the expected result?

Instructions work

What happens instead?

$ cat args.gn
is_component_build = false
is_debug = false
is_msan = true
msan_track_origins = 2
target_os = "chromeos"
use_ozone = false
use_prebuilt_instrumented_libraries = true
$ ninja aura_unittests
[...]/third_party/instrumented_libraries/binaries/msan-chained-origins-trusty.tgz: Cannot open: No such file or directory
tar (child): Error is not recoverable: exiting now
tar: Child returned status 2
tar: Error is not recoverable: exiting now
Traceback (most recent call last):
  File "../../third_party/instrumented_libraries/scripts/unpack_binaries.py", line 42, in <module>
    sys.exit(main(*sys.argv[1:]))
  File "../../third_party/instrumented_libraries/scripts/unpack_binaries.py", line 29, in main
    target_dir])
  File "/usr/lib/python2.7/subprocess.py", line 540, in check_call
    raise CalledProcessError(retcode, cmd)

I needed to do this to get the prebuilts to download:

$ GYP_DEFINES='use_prebuilt_instrumented_libraries=1 msan=1 msan_track_origins=2' src/third_party/instrumented_libraries/scripts/download_binaries.py

But I shouldn't have had to figure that out, there should be an easier way to download the prebuilts.