Usage: roll-dep third_party/WebKit 12345

That will update DEPS with the new revision, and leave the DEPS file
dirty.  It's the up to the user to 'git add; git commit;
git cl upload'.

This script won't handle arbitrary python syntax, but it will handle
variable references like Var("webkit_revision") correctly.

R=mmoss@chromium.org, iannucci@chromium.org
BUG=341098

Review URL: https://codereview.chromium.org/318153003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@275875 0039d316-1c4b-4281-b951-d872f2087c98