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}

content/child/blink_platform_impl.cc

@@ -827,6 +827,8 @@ const DataResource kDataResources[] = {
     ui::SCALE_FACTOR_NONE },
   { "HTMLMarqueeElement.js", IDR_PRIVATE_SCRIPT_HTMLMARQUEEELEMENT_JS,
     ui::SCALE_FACTOR_NONE },
+  { "PluginPlaceholderElement.js",
+    IDR_PRIVATE_SCRIPT_PLUGINPLACEHOLDERELEMENT_JS, ui::SCALE_FACTOR_NONE },
   { "PrivateScriptRunner.js", IDR_PRIVATE_SCRIPT_PRIVATESCRIPTRUNNER_JS,
     ui::SCALE_FACTOR_NONE },
 #ifdef IDR_PICKER_COMMON_JS