Improve development workflow with built libchromiumcontent

- 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.

.gitignore

@@ -41,3 +41,4 @@ node_modules/
 /vendor/node/deps/node-inspect/.npmrc
 /vendor/npm/
 /vendor/python_26/
+.gclient_done