More cr.ui.FocusRow simplifications
cr.ui.FocusRow no longer inherits from HTMLDivElement. Instead, it takes
a |root| argument in its constructor and uses |this.root| more.
Before there was lots of type variance. Something would start as a
<div>, then get decorated, but only work because cr.ui.FocusGrid derives
a <div>. This is clearer, more future-proof solution.
BUG=425626,518053
R=hcarmona@chromium.org,rdevlin.cronin@chromium.org
Review URL: https://codereview.chromium.org/
1282043004
Cr-Commit-Position: refs/heads/master@{#343333}