ninja: move bundle logic out of WriteTarget
ninja: move bundle logic out of WriteTarget

The conceptual logic at the end of WriteSpec now is:
  final_output = linker_output = WriteTarget()
  if is_bundle:
    final_output = WriteBundle()
  return final_output

That is, WriteBundle is an additional optional final step in the main
function that sequences actions, compiles, etc.

Review URL: http://codereview.chromium.org/9190068
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp