DevTools: [CodeMirror] roll xml mode

This patch rolls xml codeMirror mode to @7f82421f6. New version supports
value-less attributes in html code.

TBR=vsevik

Review URL: https://codereview.chromium.org/22344003

git-svn-id: svn://svn.chromium.org/blink/trunk@155757 bbb929c8-8fbe-4397-9dbb-9b2b20218538