The standalone (non-Chromium) WebRTC build has some targets
that on the Mac use Objective-C features that require the
targets to be deployed to 10.7 or newer hosts (i.e., they
won't run on 10.6).

Because we share GN //build configurations between the WebRTC
and Chromium project, we need to give WebRTC to toggle the
minimum configuration needed; we do this by creating a new file called
build.gni in //build_overrides/build.gni file, and
making //build/config/mac/mac_sdk.gni import that. This introduces
a dependency from //build to //build_overrides, and means that other
projects that depend on //build will need to create a new file when
this change is rolled in..

R=brettw@chromium.org
BUG=webrtc:5453, 588513

Review URL: https://codereview.chromium.org/1686373002

Cr-Commit-Position: refs/heads/master@{#376856}