Add PluginPlaceholderElement.js to Blink platform resources.

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
Committed: https://crrev.com/7424a1b1582d06cee76e259080c2062e299662fa
Cr-Commit-Position: refs/heads/master@{#294884}

[jbroman, 2014-09-15 12:59:23]

jbroman@chromium.org changed reviewers:
+ jochen@chromium.org

[jochen (gone - plz use gerrit), 2014-09-15 14:25:33]

lgtm

[jbroman, 2014-09-15 18:57:03]

The CQ bit was checked by jbroman@chromium.org

[commit-bot: I haz the power, 2014-09-15 19:17:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/556793006/1

[commit-bot: I haz the power, 2014-09-15 20:32:25]

Committed patchset #1 (id:1) as 281a925842612a662c067c504beb25fbe63b55ee

[commit-bot: I haz the power, 2014-09-15 20:35:24]

Patchset 1 (id:??) landed as
https://crrev.com/7424a1b1582d06cee76e259080c2062e299662fa
Cr-Commit-Position: refs/heads/master@{#294884}