Rename childNodeCount() / childNode() methods for clarity

Rename childNodeCount() / childNode() methods for clarity and to avoid bad
uses. They are now called countChildren() / traverseToChildAt() to make it
clear that these are doing work and are now simpler getters.

Also rename hasChildNodes() to hasChildren() for consistency.

R=eseidel, adamk

Review URL: https://codereview.chromium.org/171773008

git-svn-id: svn://svn.chromium.org/blink/trunk@167616 bbb929c8-8fbe-4397-9dbb-9b2b20218538