Refactor reference filters in preparation for filter effects region.

Currently, the CPU rendering path for reference filters builds
FilterEffects which have the FilterEffectsRenderer (filter) as their
parent. The GPU path by contrast builds actual ReferenceFilters for
them. Changing the CPU path to do the same moves us closer to being
able to implement filter effects region (since it's a per-Filter
property).  It also means that we can centralize page zoom scaling,
remove the silly inverse-scaling that's applied to shorthand filters,
and remove the transform from the FilterEffectsRenderer.

BUG=240827

Review URL: https://codereview.chromium.org/635843002

git-svn-id: svn://svn.chromium.org/blink/trunk@183496 bbb929c8-8fbe-4397-9dbb-9b2b20218538