- Add `--debug_libchromiumcontent` to build libchromiumcontent for debugging
(shared library build).
- By default, only invoke `gclient sync` the first time to checkout chromium
source tree. Add `--force_update_libchromiumcontent` switch to force updating.
- Document new options.
The goal is to allow faster edit/compile cycles when debugging/making changes
to libchromiumcontent.
