DevTools: [Elements] Highlight HTML entities in the DOM tree

Otherwise, HTML entities look exactly the same as the corresponding
character sequences in text nodes.

R=pfeldman
BUG=299147

Review URL: https://codereview.chromium.org/66753002

git-svn-id: svn://svn.chromium.org/blink/trunk@162822 bbb929c8-8fbe-4397-9dbb-9b2b20218538