Make RenderWidgetHostViewQtDelegate a pure interface.

RenderWidgetHostViewQtDelegate acts as a bidirectional interface to
avoid exporting Chromium symbols outside of the core dynamic library.

The problem is that, other than this, from the top layer point of
view, its responsibilities are the same as RenderWidgetHostViewQt,
and it would be better not to split its logic without properly
defined responsibilities.

Using it as a base class and interfacing through its protected
methods is also cumbersome and make the destination of calls on the
upper layer difficult to discern.

Use instead a dual pure interface mechanism like WebContentsAdapter
and pass the callback client interface directly in
WebContentsAdapterClient::CreateRenderWidgetHostViewQtDelegate.
This allows RenderWidgetHostViewQtDelegate to be solely what it
should be, an interface.

Change-Id: I4e55439ae7f9539cc9e360f0756fbf391405f3b7
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>