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As it is possible to specify multiple value for target_os in
.gclient, update the script build/mac_toolchain.py to download
the file for the hermetic build on all those OSes. Change the
path where the hermetic files are downloaded to use different
path for each OS.

BUG=680069

Review-Url: https://codereview.chromium.org/2626063002
Cr-Commit-Position: refs/heads/master@{#443198}