Add svn:ignore on dirs that shouldn't be wiped during gclient revert

This will speed up our buildbots and trybots since it also enables
incremental builds again (currently out/ was wiped for each build).

The Chromium toolchain somehow has changed behavior, since the out/
dir didn't use to be wiped. This change should get faster builds
back again.

git-svn-id: 4adac7df-926f-26a2-2b94-8c16560cd09d