Move screenColorProfile from Platform to ImageDecoder

Platform is the wrong place for screenColorprofile, where it was
the only method there that did not know its Widget. ImageDecoder
is the only caller currently, move it there.

No change in behavior, covered by existing layout tests.

BUG=369787

Review URL: https://codereview.chromium.org/570023002

git-svn-id: svn://svn.chromium.org/blink/trunk@181978 bbb929c8-8fbe-4397-9dbb-9b2b20218538