DevTools: Add CSS.getPlatformFontsForNode in protocol.json

This patch adds method CSS.getPlatformFontsForNode to the protocol.
The method returns platform fonts that were used to render child text
nodes for the given node.

BUG=135489

Review URL: https://chromiumcodereview.appspot.com/22923010

git-svn-id: svn://svn.chromium.org/blink/trunk@156505 bbb929c8-8fbe-4397-9dbb-9b2b20218538