Make WebView::close not crash and start to fix navigation in SkyShell

Make WebView::close not crash and start to fix navigation in SkyShell

However WebView::close() no longer crashes.  close() is never called
in MojoShell since mojo can't shutdown yet.

I tried closing the existing WebView and replacing it
but somehow that caused it to only draw red.  After a while
of looking at this with abarth we decided to just load into
the same WebView for now.

Eventually we should do something smarter where we start the
provisional load and only replace the webview once the new one is
ready, but that's a later CL.

R=abarth@chromium.org
BUG=

Committed: https://chromium.googlesource.com/external/mojo/+/40c5d6474f79ce40036ef1d9780a5dc7b8c5f0ce

[abarth-chromium, 2015-02-25 23:21:32]

https://codereview.chromium.org/952273003/diff/1/sky/shell/ui/engine.cc
File sky/shell/ui/engine.cc (right):

https://codereview.chromium.org/952273003/diff/1/sky/shell/ui/engine.cc#newco...
sky/shell/ui/engine.cc:154: LoadURL(request->url);
We should do this asynchronously.

[abarth-chromium, 2015-02-25 23:25:11]

lgtm

[eseidel, 2015-02-25 23:25:22]

Committed patchset #2 (id:20001) manually as
40c5d6474f79ce40036ef1d9780a5dc7b8c5f0ce (presubmit successful).