Allow mojo_debug to run on mac

BUG=
R=ppi@chromium.org

Review URL: https://codereview.chromium.org/1929943002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 1e5279fc6e61aa6172a8376273c366a5104f2a49

mojo_debug

diff --git a/mojo_debug b/mojo_debug
index dc1000b..cf24971 100755
--- a/mojo_debug
+++ b/mojo_debug
@@ -7,6 +7,7 @@
 import codecs
 import logging
 import os.path
+import platform
 import requests
 import signal
 import subprocess
@@ -194,8 +195,7 @@
       # TODO(etiennej): Always select the most recent toolchain?
       'arm-linux-androideabi-4.9',
       'prebuilt',
-      # TODO(etiennej): DEPS mac NDK and use it on macs.
-      'linux-x86_64',
+      '%s-%s' % (platform.system().lower(), platform.machine()),
       'bin',
       'arm-linux-androideabi-gdb')
   python_gdb_script_path = os.path.join(os.path.dirname(__file__),