Add an API to request the application brings a webview in to the foreground

[Chris Coulson]

We should expose a webview signal that fires off of WebContentsDelegate::ActivateContents() - the use case for this is that if a user action results in a navigation in another window (eg, clicking on a <a> element with a target attribute), there should be a mechanism to request that the webview hosting the target window is focused.

There may be another use case for this - focusing a webview that has a pending alert dialog.