[Tools] Ignore more directories in .gclient-xwalk.

Add more directories to the default list of ignored directories in
generate_gclient-xwalk.py to avoid spending time and bandwidth downloading
things that are not relevat for Crosswalk, such as a custom depot_tools
checkout (one already needs one to download Crosswalk in the first place),
Chromium buildbot stuff, the commit-queue source code and ChromeOS
dependencies.

(cherry picked from commit d6186d1cf5d6f0a34af34327b1c1194527503a4c)