[scenic] web_view: view_provider->base_view

All topaz clients expose both the V1 and V2 ViewProvider interfaces,
prepping them for the full move to eventpairs.

No dependencies remain on ui/view_framework in web_view or web_runner.

TEST: set_root_view web_view

SCN-898 #comment
SCN-993 #comment

Change-Id: Iaeaa3deaaad30148a102c0fec086788a02d4f97f
10 files changed