Make `window.visualViewport` match IDL from spec.

`window.visualViewport` should never return null. I've made the change
to the IDL to clarify that and also prevent returning nullptr from
LocalDOMWindow::visualViewport.

I also renamed view_ to visualViewport_ in LocalDOMWindow as I missed
this in my renaming patch https://crrev.com/c/546756/

Bug: 595826
Change-Id: I24e009e2b2ae0ecfbad66ccf565c03dcea7df24d
Reviewed-on: https://chromium-review.googlesource.com/550262
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483012}
5 files changed