Add a comment explaining what RoundedRect is

...and why it uses integer units.

R=eae@chromium.org
BUG=

Review URL: https://codereview.chromium.org/48833006

git-svn-id: svn://svn.chromium.org/blink/trunk@160762 bbb929c8-8fbe-4397-9dbb-9b2b20218538