[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.
Merge pull request #718 from rakuco/ignore-more-directories

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