Get the proper Mac SDK path for the GN build.

This gets the path from xcodebuild rather than hardcoding it in the .gn files.

R=thakis
BUG=

Review URL: https://codereview.chromium.org/136793020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245403 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245403