Reland #2: Switch the default build configuration to use the 10.10 SDK.

Reland #2: Switch the default build configuration to use the 10.10 SDK.

The first reland caused Webkit layout test errors on 10.8 and 10.9. Those
errors have been fixed.

Original issue's description:
> mac: Switch the default build configuration to use the 10.10 SDK.
>
> This change is intended to be temporary. The goal is to release a Canary
> compiled against the 10.10 SDK, and then to revert this CL.
>
> BUG=463170
>
> Committed: https://crrev.com/42ba22e0b8e87f758804537f64db23ff6438e451
> Cr-Commit-Position: refs/heads/master@{#340992}

TBR=thakis@chromium.org,amineer@chromium.org
BUG=463170, 515836
Committed: https://crrev.com/a9cbe267748e0360f8c994a1c7209f86f33b944f
Cr-Commit-Position: refs/heads/master@{#342870}

[erikchen, 2015-08-11 17:40:00]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-08-11 17:40:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1268803004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1268803004/1

[commit-bot: I haz the power, 2015-08-11 19:31:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-11 19:32:38]

Patchset 1 (id:??) landed as
https://crrev.com/a9cbe267748e0360f8c994a1c7209f86f33b944f
Cr-Commit-Position: refs/heads/master@{#342870}