Once created, keep the WebEngineContext until shutdown.

Chromium isn't expecting the destruction and reconstruction of the
ContentMainRunner and BrowserMainRunner. This will need a few tweaks
before we can do this without hitting asserts in debug builds.

Change-Id: I0c46a2db27350b3f6352e3eb8a5ec7ecff06784e
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>