1. Handle the case of empty embedder_extension_id in guestview.h.

1. Handle the case of empty embedder_extension_id in    guestview.h.

2. Introduce a template class GuestView<T> to handle the    *guestview-specific functions in guestview.h:
   . Rename GuestView to GuestViewBase, introduce GuestView<T> class, and move *ViewGuest-specific functions to     GuestView<T>:
        -- FromWebContents<T>(...)
        -- From<T>(...).
   . Introduce a template member function As() in      GuestViewBase to replace all of the As*View functions.

3. In each derived *ViewGuest class , e.g., WebViewGuest &    AdViewGuest, define a static variable to show its view type    (string).

BUG=351824

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=266297

Review URL: https://codereview.chromium.org/237533008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267079 0039d316-1c4b-4281-b951-d872f2087c98