Remove viewportDefiningElement.

Remove viewportDefiningElement.

This was used for quirky scrollbar behavior on the web related to
which element defined in the viewport (body, html, etc.). In Sky we plan to
allow multiple elements as direct children of the Document so we need to get
rid of checks like this. The good news is this already does nothing in sky
because the root never has scrollbars automatically.

R=ojan@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/5c9518c84b3641e6a1f804446a360f9835ab1779

[ojan, 2015-01-17 02:34:41]

lgtm

[esprehn, 2015-01-17 02:35:15]

Committed patchset #1 (id:1) manually as
5c9518c84b3641e6a1f804446a360f9835ab1779 (presubmit successful).