DevTools: Use CodeMirror modes for syntax highlighting

Relanding https://codereview.chromium.org/22638008 which was reverted
at r156210 due to breaking styles sidebar. This patch fixes
style class naming from webkit-css-property into cm-css-property.

BUG=260589

Review URL: https://chromiumcodereview.appspot.com/23283017

git-svn-id: svn://svn.chromium.org/blink/trunk@156793 bbb929c8-8fbe-4397-9dbb-9b2b20218538