Create a dummy resource for DocumentExecCommand.js

This prepares for the landing of
http://crrev.com/530663002

which now requires a three-sided change due to changes in how Blink
accesses private script resources.

Part 2: http://crrev.com/588473003

BUG=408887
TEST=n/a; no code changes

Review URL: https://codereview.chromium.org/583753003

git-svn-id: svn://svn.chromium.org/blink/trunk@182312 bbb929c8-8fbe-4397-9dbb-9b2b20218538