Add IsOSElCapitan() and other util functions.

Add IsOSElCapitan() and other util functions.

These functions mirror the IsOSYosemite(), etc. functions that already
exist, and are needed for at least one future cl.

BUG=510587
Committed: https://crrev.com/49c9add371f8e802e1788adfa0a2012e1e4d3064
Cr-Commit-Position: refs/heads/master@{#339085}

[shrike, 2015-07-15 21:21:39]

shrike@chromium.org changed reviewers:
+ thakis@chromium.org

[shrike, 2015-07-16 16:45:10]

PTAL

[Nico, 2015-07-16 16:52:42]

lgtm

[shrike, 2015-07-16 17:29:56]

The CQ bit was checked by shrike@chromium.org

[commit-bot: I haz the power, 2015-07-16 17:30:43]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1231803007/1

[commit-bot: I haz the power, 2015-07-16 18:40:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-16 18:42:12]

Patchset 1 (id:??) landed as
https://crrev.com/49c9add371f8e802e1788adfa0a2012e1e4d3064
Cr-Commit-Position: refs/heads/master@{#339085}