--no-pager looks ugly in logs and output and makes commands harder to
read. Setting an environment variable is better.

This is a followup to https://chromiumcodereview.appspot.com/14104005/
which added --no-pager.

Review URL: https://chromiumcodereview.appspot.com/18173003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@210503 0039d316-1c4b-4281-b951-d872f2087c98