Delete tooltip explaining color coding in the heap snapshot view

The tooltip content is rather confusing. Given that the color scheme is pretty obvious and that it is described in the documentation we decided to delete that tooltip.

BUG=None

Review URL: https://chromiumcodereview.appspot.com/22961005

git-svn-id: svn://svn.chromium.org/blink/trunk@156219 bbb929c8-8fbe-4397-9dbb-9b2b20218538