Use the new nacl_core_sdk mode for package_version within Chromium.

A new "nacl_core_sdk" mode has been introduced for NaCl toolchain
packages which allows the NaCl tree to specify what external facing
users of the toolchain should download. Currently it only filters out
the arm_trusted package which is not useful for Chromium. Eventually we
will also use this to filter out a set of "raw" toolchains which do not
contain the core_sdk.

R=bradnelson@chromium.org, dschuff@chromium.org
BUG=None
TEST=trybots

Review URL: https://codereview.chromium.org/735163002

Cr-Original-Commit-Position: refs/heads/master@{#305102}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 24988063e1b1233b450eb3756dd40b09780872b5

download_nacl_toolchains.py

@@ -50,6 +50,7 @@ def Main(args):
   if 'target_arch=arm' not in os.environ.get('GYP_DEFINES', ''):
       args.extend(['--exclude', 'nacl_arm_newlib'])
 
+  args.extend(['--mode', 'nacl_core_sdk'])
   args.append('sync')
   package_version.main(args)