Move build_dir_prefix up a level of nesting so we don't create a 'variables' configuration in xcode.
Move build_dir_prefix up a level of nesting so we don't create
a 'variables' configuration in xcode.

BUG=99226


Review URL: http://codereview.chromium.org/8164001

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@104321 0039d316-1c4b-4281-b951-d872f2087c98

ropik/chromium,yitian134/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,yitian134/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,adobe/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,ropik/chromium,adobe/chromium