Update layout test lang-glyph-cache-separation.html

Follow-up of blink r158398.
- Use U+76F4 as the testing character
- Now covers zh-CN, zh-TW, ja and ko.

BUG=

Review URL: https://codereview.chromium.org/25267004

git-svn-id: svn://svn.chromium.org/blink/trunk@158642 bbb929c8-8fbe-4397-9dbb-9b2b20218538