Switch the Mac build from 32-bit to 64-bit.

Switch the Mac build from 32-bit to 64-bit.

As of M39, Chrome is a 64-bit only binary. This changes the default build
configuration to 64-bit globally. After this CL, the 32-bit Mac build is no
longer supported.

BUG=405542, 18323
R=mark@chromium.org

Committed: https://chromium.googlesource.com/chromium/src/+/602b4bf2748b43b9cb697b039a35131b9e0c913c

[Robert Sesek, 2014-11-17 21:34:44]

I thought about hard-coding x64 for Mac, but since any Mac that reports ia32
host_arch won't be able to link chrome_dll anyways, I decided to keep the GYP
conditions simpler.

[Mark Mentovai, 2014-11-17 22:30:33]

LGTM

[commit-bot: I haz the power, 2014-11-20 18:39:07]

Patchset 2 (id:??) landed as
https://crrev.com/602b4bf2748b43b9cb697b039a35131b9e0c913c
Cr-Commit-Position: refs/heads/master@{#305043}

[Robert Sesek, 2014-11-20 18:39:57]

Committed patchset #2 (id:20001) manually as
602b4bf2748b43b9cb697b039a35131b9e0c913c (presubmit successful).