Remove Node::hasRenderer()

This patch removes Node::hasRenderer() which was added in [1] because
it is not needed and adds complexity to the Node interface.

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=162653.

Review URL: https://codereview.chromium.org/300623002

git-svn-id: svn://svn.chromium.org/blink/trunk@174753 bbb929c8-8fbe-4397-9dbb-9b2b20218538