Mac: Let compile.py automatically pick ninja over xcodebuild based on timestamp.
Mac: Let compile.py automatically pick ninja over xcodebuild based on timestamp.

xcode is still the default gyp generator, so this will have no effect in
practice for now. But once the generator default is changed, things should
Just Work for the compile.py change. zip_build and friends will then likely
zip up the wrong folder; that will be addressed in a follow-up.

Also remove similar code for linux that was added for the make->ninja
transition, which is now complete.

Also set env vars CHROMIUM_BUILD, OFFICIAL_BUILD, CHROME_BUILD_TYPE for
xcode builds. These are apparently not used in mac builds, but make, ninja,
and msvs all set these variables, so set them on mac/ios too for consistency.

BUG=239257,294387

Review URL: https://chromiumcodereview.appspot.com/24249002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@226352 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build