Do not enable scripting in WebHelperPluginImpl page.

According to https://bugs.webkit.org/attachment.cgi?id=143842&action=review#line240,
this was required to avoid a crash during plugin creation. The existing code
says it must be enabled in ScriptController::windowScriptNPObject().
I can't find a reason for that, and it works fine without scripting.

It is better to not allow scripting, especially since we do not need it.

Review URL: https://chromiumcodereview.appspot.com/21994003

git-svn-id: svn://svn.chromium.org/blink/trunk@155462 bbb929c8-8fbe-4397-9dbb-9b2b20218538