Reland #1: Switch the default build configuration to use the 10.10 SDK.

Reland #1: Switch the default build configuration to use the 10.10 SDK.

All known problems have been fixed since the first attempt to land this CL.

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
Committed: https://crrev.com/345c0cf47947f209e628941b38d76dca90aee49d
Cr-Commit-Position: refs/heads/master@{#341261}

[erikchen, 2015-07-31 00:22:28]

The CQ bit was checked by erikchen@chromium.org

[commit-bot: I haz the power, 2015-07-31 00:23:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1257643005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1257643005/1

[commit-bot: I haz the power, 2015-07-31 01:31:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-31 01:32:27]

Patchset 1 (id:??) landed as
https://crrev.com/345c0cf47947f209e628941b38d76dca90aee49d
Cr-Commit-Position: refs/heads/master@{#341261}

[dmurph, 2015-07-31 10:06:57]

A revert of this CL (patchset #1 id:1) has been created in
https://codereview.chromium.org/1266603007/ by dmurph@chromium.org.

The reason for reverting is: I believe this is causing a lot of issues with
layout, no idea why though.
https://code.google.com/p/chromium/issues/detail?id=515836.