Use Traversal<>::firstChild() instead of firstWithin() when iterating over children

Use Traversal<>::firstChild() instead of firstWithin() when iterating over
children. It makes it clearer that we are only iterating over the children and
not traversing the descendants.

There is no behavior change since firstChild() is equivalent to firstWithin()
for ElementTraversal / Traversal<HTMLElement>. However, using firstChild()
improves readability a bit.

Review URL: https://codereview.chromium.org/412033004

git-svn-id: svn://svn.chromium.org/blink/trunk@178819 bbb929c8-8fbe-4397-9dbb-9b2b20218538