Stop pretending to manage the color table in DiscardablePixelRef.

We never have a non-null color table in Chromium/Blink, this feature is
deprecated in Skia, and the previous code to manage it did not correctly
balance refs and unrefs.

Instead, we'll just assert that it's always null.

BUG=

Review URL: https://chromiumcodereview.appspot.com/21720005

git-svn-id: svn://svn.chromium.org/blink/trunk@155569 bbb929c8-8fbe-4397-9dbb-9b2b20218538