Convert all Objective-C code in following build files to ARC:

./chrome/app/BUILD.gn
./chrome/app/resources/BUILD.gn
./chrome/app/strings/BUILD.gn
./chrome/app/theme/BUILD.gn
./chrome/browser/BUILD.gn
./chrome/browser/signin/BUILD.gn
./chrome/browser/ui/omnibox/BUILD.gn
./chrome/browser/variations/BUILD.gn
./chrome/browser/voice/BUILD.gn
./chrome/BUILD.gn
./chrome/common/app_group/BUILD.gn
./chrome/common/BUILD.gn
./chrome/common/physical_web/BUILD.gn
./chrome/share_extension/BUILD.gn
./chrome/share_extension/strings/BUILD.gn
./chrome/test/BUILD.gn
./chrome/today_extension/BUILD.gn
./chrome/today_extension/strings/BUILD.gn
./chrome/tools/strings/BUILD.gn