With a fix: actually set dry_run for presubmit, and don't set it for other builders.

R=machenbach@chromium.org,sergiyb@chromium.org
BUG=601128,594127,591030

Review-Url: https://codereview.chromium.org/1926663002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@300295 0039d316-1c4b-4281-b951-d872f2087c98