wpe: Reintroduce persistent WebContext
A WebContext leak was introduced in MR gst-plugins-bad!2252 (merged). If we wanted one WebContext per WebView we should also unref the WebKitWebContext when destroying the WebView.
This patch reintroduces the persistent WebContext, initially part of gst-plugins-bad!1484 (merged).
Fixes #1084 (closed)
Edited by Philippe Normand