Remove the Mac-specific path for getting the color space in ImageDecoder.

This path required a connection to the WindowServer, which should be avoided
(see the two bugs for details). Instead this data can be retrieved via the
ColorProfile class, which can access a value that has been cached during
pre-sandbox warmup.

BUG=397642,306348
R=noel@chromium.org

Review URL: https://codereview.chromium.org/554033002

git-svn-id: svn://svn.chromium.org/blink/trunk@181761 bbb929c8-8fbe-4397-9dbb-9b2b20218538