mac: Switch from -O3 to -O2.

mac: Switch from -O3 to -O2.

All other platforms use other -O2 or -Os. Since Linux even uses the same
compiler as mac now, there isn't a great reason for Mac to use a different
optimization level.

In a static release build, this reduces binary size from 175.1MB to 174MB, so
it's probalby not a huge change anyways. If it is, the perf bots will tell us.

BUG=none

Committed: https://crrev.com/9ce1b7510648285b80ff75213384bf4186b8c9dd
Cr-Commit-Position: refs/heads/master@{#303476}

[Nico, 2014-11-08 00:00:23]

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

[Mark Mentovai, 2014-11-10 13:38:18]

LGTM.

-O3 was chosen with input from performance tests a long time ago on a very
different compiler. I’m interested to see what impact, if any, this change has
on performance.

[Nico, 2014-11-10 15:18:21]

The CQ bit was checked by thakis@chromium.org

[commit-bot: I haz the power, 2014-11-10 15:19:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/714433002/1

[commit-bot: I haz the power, 2014-11-10 16:19:47]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-10 17:12:50]

Patchset 1 (id:??) landed as
https://crrev.com/9ce1b7510648285b80ff75213384bf4186b8c9dd
Cr-Commit-Position: refs/heads/master@{#303476}