Handle PlatformPageClient not being set in Windows PluginView.

The helper contentsToNativeWindow assumed that the PlatformPageClient
would always be set, which caused problems with plugin content in head-
less client applications such as CutyCapt.

Task-number: QTBUG-35224
Change-Id: Iadce5fb62bd45dd0d1b15e0d7918cfbbff1b3fae
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>