DevTools: fix InspectorCSSAgent::getComputedStyleForNode

This patch fixes InspectorCSSAgent::getComputedStyleForNode so that it
starts to handle not only elements but all nodes. This perfectly
aligns with the current protocol.json description.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23045006

git-svn-id: svn://svn.chromium.org/blink/trunk@156239 bbb929c8-8fbe-4397-9dbb-9b2b20218538