Remove ComposedShadowTreeWalker::fromFirstChild

Nothing uses fromFirstChild, and nothing should use it since its not aware of pseudo elements,
so lets just remove it.

Review URL: https://chromiumcodereview.appspot.com/22562006

git-svn-id: svn://svn.chromium.org/blink/trunk@155726 bbb929c8-8fbe-4397-9dbb-9b2b20218538