Add ARM trusted toolchain to download_toolchains invocation.
Add ARM trusted toolchain to download_toolchains invocation.

Adding the ARM trusted toolchain option ot download_toolchains will fix and
issue where the trusted ARM toolchain was beign downloaded every time you run
gclient sync.
Review URL: http://codereview.chromium.org/7857035

git-svn-id: e22621a84199d42b34715cdc089545aeedbbf23f@1128 050acbb0-2703-11df-ab0a-9f3f633ae91d

sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk,sbc100/nativeclient-sdk