Create a dummy resource for PluginPlaceholderElement.js.

This prepares for the landing of
  https://codereview.chromium.org/522783002/

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

Part 2: https://codereview.chromium.org/556793006/

BUG=364716

Review URL: https://codereview.chromium.org/570863002

git-svn-id: svn://svn.chromium.org/blink/trunk@181980 bbb929c8-8fbe-4397-9dbb-9b2b20218538