DevTools: fix protocol.json Page.colorPicked event

The event should report DOM.RGBA color instead of string.

R=pfeldman@chromium.org, pfeldman

Review URL: https://codereview.chromium.org/478943002

git-svn-id: svn://svn.chromium.org/blink/trunk@180409 bbb929c8-8fbe-4397-9dbb-9b2b20218538