The --extra_patchlevel flag was added in
https://codereview.chromium.org/1988583002/

In order to make it possible for WebRTC patches to apply in
src/third_party/webrtc of a Chromium checkout, one directory
level needs to be trimmed.

BUG=438952, 611808

Review-Url: https://codereview.chromium.org/1982243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300618 0039d316-1c4b-4281-b951-d872f2087c98