Improve consistency of nextNodeWithGreaterTabIndex() and previousNodeWithLowerTabIndex().

* nextNodeWithGreaterTabIndex:
  Set |currentTabIndex| to |winningTabIndex|
  They are same in the |if| block.  However |currentTabIndex| looks better.

* previousNodeWithLowerTabIndex:
  Remove isNonKeyboardFocusableShadowHost() check.  It's redandunt because
  shouldVisit() contains it.

BUG=
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/700313002

git-svn-id: svn://svn.chromium.org/blink/trunk@184889 bbb929c8-8fbe-4397-9dbb-9b2b20218538