[Refactor Xcode Objects] Enable adding per file '--help' compiler flag.

This CL refactors xcode_object.h and xcode_object.cc to enable generating '--help'
as per file compiler flag for the files in the "Compiler Sources" in
"Build Phases".

BUG=614818

Review-Url: https://codereview.chromium.org/2574333002
Cr-Original-Commit-Position: refs/heads/master@{#439913}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8fb7d9d9c9ddf8bd3d1448241fafa27416e1f756
3 files changed
tree: b346211122a4bdc98d0993d99548d73422965813
  1. tools/