Add the named color `rebeccapurple` (#639)

http://dev.w3.org/csswg/css-color-4/#valuedef-rebeccapurple

Contributed by Mathias Bynens <mathiasb@opera.com>;.

BUG=384254
TBR=esprehn

Review URL: https://codereview.chromium.org/432773002

git-svn-id: svn://svn.chromium.org/blink/trunk@179321 bbb929c8-8fbe-4397-9dbb-9b2b20218538