Fix chromium trunk url in DEPS.

Makes "svn/trunk" part of the variable so that it can be replaced with "chrome/trunk" when using mirrors.

BUG=372738
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/289473006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21300 ce2b1a6d-e550-0410-aec6-3dcde31c8c00