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

Review URL: https://codereview.chromium.org/1268803004

Cr-Commit-Position: refs/heads/master@{#342870}