[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.