Introduce WebContentsViewEflDelegate interface.

Introduce WebContentsViewEflDelegate interface.

The idea is to avoid the need to modify upstrem WebContentsViewDelegate
interface. WebContentsViewEflDelagate is supposed to hold only extra EFL
specific extensions we'd otherwise have to put in the non Efl
counterpart of the interface. The downside is WebCotnentsViewEfl
implementation has two delegates now and care has to be taken to call
function from the correct one. Still it's probably preferable to do it
this way instead of patching chromium.

Change-Id: I6cdb0c966b11be33bf71ab3f45f172259cf063ae
Signed-off-by: Piotr Tworek <p.tworek@samsung.com>