Add debug printers to DisplayItems

This patch adds debug printers for debugging DisplayItems. Example output:
{nodeName: "DIV", renderer: "0x3fa22f98c210", type: "DrawingPaintPhaseChildOutlines", bounds: [...]}

Review URL: https://codereview.chromium.org/688283003

git-svn-id: svn://svn.chromium.org/blink/trunk@184680 bbb929c8-8fbe-4397-9dbb-9b2b20218538