Add showNode convenience function for debuggers

This patch improves our debugger convenience functions by adding the new
showNode(const Node*) function. Debuggers can now type:
  showNode(renderer->node())
and get output like:
  DIV 0x25808590 ID="doge" CLASS="foo bar" STYLE="color: honeydew"

Previously, the following would have been printed:
  DIV 0x25808590 CLASS=foo bar STYLE=color: honeydew

This patch also adds the id attribute to Node::showNode and quotes the
attribute output. These changes will appear when calling showNode as well
as its friends (showTreeForThis, showTreeForThisAcrossFrame, etc).

Review URL: https://codereview.chromium.org/190793008

git-svn-id: svn://svn.chromium.org/blink/trunk@168785 bbb929c8-8fbe-4397-9dbb-9b2b20218538