Have HTMLTableRowsCollection::item() return an HTMLTableRowElement*

Have HTMLTableRowsCollection::item() return an HTMLTableRowElement* instead of
an Element* as HTMLTableRowsCollection only contains HTMLTableRowElements. This
gets rid of an unnecessary isHTMLTableRowElement() check in AXTable.

R=adamk@chromium.org

Review URL: https://codereview.chromium.org/404403002

git-svn-id: svn://svn.chromium.org/blink/trunk@178675 bbb929c8-8fbe-4397-9dbb-9b2b20218538