Set the WebViewDelegate earlier during WebView creation to ensure that...

Set the WebViewDelegate earlier during WebView creation to ensure that
DidCreateDataSource gets called when constructing the initial data source
for the WebView's main frame.

This ensures that the NavigationState associated with a WebDataSource
can never be null.  I went ahead and removed some of the null-checks
that were added to avoid other related crashes.  (To fix bug 15594, I
really really didn't want to add another one of those null checks.)

BUG=15594
TEST=partially covered by existing ui tests
R=dglazkov