cr: Add support for mac

cr: Add support for mac

Make it possible to use cr for building on mac. Mostly this is a clone of what
we are doing on Linux, except that I had to work around "readlink -e" not being
available.

Committed: https://crrev.com/a5f61e43cea94e305ecc9838da95e4fd9885f891
Cr-Commit-Position: refs/heads/master@{#314336}

[Sami, 2015-02-03 13:56:12]

skyostil@chromium.org changed reviewers:
+ perezju@chromium.org

[perezju, 2015-02-03 14:07:43]

rubberstamp lgtm

[Sami, 2015-02-03 14:24:45]

The CQ bit was checked by skyostil@chromium.org

[commit-bot: I haz the power, 2015-02-03 14:25:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/898683003/1

[commit-bot: I haz the power, 2015-02-03 15:09:13]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-03 15:10:39]

Patchset 1 (id:??) landed as
https://crrev.com/a5f61e43cea94e305ecc9838da95e4fd9885f891
Cr-Commit-Position: refs/heads/master@{#314336}