Move color chooser objects to the Oilpan heap.

Keep these on the heap so that their raw pointers to heap objects can be
correctly traced.

R=mkwst,haraken,tkent,keishi
BUG=340522

Review URL: https://codereview.chromium.org/631833002

git-svn-id: svn://svn.chromium.org/blink/trunk@183538 bbb929c8-8fbe-4397-9dbb-9b2b20218538