This makes the character escaping in the graph visualizer less generic
but correct against the JSON spec. The spec has the following definition
for valid characters within a double quoted string:

char ::
  any-Unicode-character-except-"-or-\-or-control-character
  \"
  \\
  \/
  \b
  \f
  \n
  \r
  \t
  \u four-hex-digits

R=bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2421313002
Cr-Commit-Position: refs/heads/master@{#40364}