This only affects the PNG image decoder, where we have already created
the color table without premultiplication. Since the RowProcChooser is
just providing a proc that chooses indices into the color table, it can
just return the same RowProc.
Update test_row_proc_choice. It was testing to ensure that we hadn't
changed the behavior from the original version of setPrefConfigTable.
In this case, we deliberately changed the behavior, so we need to
change the test.
BUG=b/12024301
Review URL: https://codereview.chromium.org/657863005