Revert of Remove unused build override (patchset #2 id:20001 of https://codereview.chromium.org/2498603002/ )

Reason for revert:
This breaks all the Mac builds.

/b/build/slave/ios-simulator/build/src/buildtools/mac/gn gen //out/Debug-iphonesimulator --check
  -> returned 1
ERROR at //build/config/mac/mac_sdk.gni:18:27: Undefined identifier
  mac_deployment_target = mac_deployment_target_build_override
                          ^-----------------------------------
See //build/toolchain/mac/BUILD.gn:14:1: whence it was imported.
import("//build/config/mac/mac_sdk.gni")
^--------------------------------------
See //BUILD.gn:73:1: which caused the file to be included.
group("gn_all") {
^----------------
GN gen failed: 1

Original issue's description:
> Remove unused build override
>
> The only downstream project using this used to be WebRTC, where it's no
> longer needed.
>
> BUG=webrtc:6431
> NOTRY=true
>
> Committed: https://crrev.com/6dd0024530bfa1d1f88840204633788bad70239e
> Cr-Commit-Position: refs/heads/master@{#433856}

TBR=dpranke@chromium.org,kjellander@chromium.org,kthelgason@webrtc.org,kthelgason@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:6431

Review-Url: https://codereview.chromium.org/2520383002
Cr-Commit-Position: refs/heads/master@{#433861}
1 file changed