Add PluginPlaceholderElement.js to Blink platform resources.

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 1: https://codereview.chromium.org/570863002/

BUG=364716

Review URL: https://codereview.chromium.org/556793006

Cr-Commit-Position: refs/heads/master@{#294884}
1 file changed