[Tools] Pass --delete_unversioned_trees to gclient sync.

Change fetch_deps.py so that we remove repositories that are no longer used;
especially when we move to different Chromium versions and the repositories
we track change.
DEPS.xwalk: Roll chromium-crosswalk (1f82149 -> 6ed4b3f).

6ed4b3f Merge pull request #133 from rakuco/ignore-ozone
94d17a7 .gitignore: Ignore ozone/.
Merge pull request #1652 from rakuco/cr-roll

DEPS.xwalk: Roll chromium-crosswalk (1f82149 -> 6ed4b3f).
Merge pull request #1647 from rakuco/delete-unversioned

[Tools] Pass --delete_unversioned_trees to gclient sync.