Fix for filter backing store size cap.

Use the device scale when capping the upper size of the filter backing
store, not the world-space size.

Also add a comment to remove this once all platforms use impl-side
painting.

BUG=400445

Review URL: https://codereview.chromium.org/446593003

git-svn-id: svn://svn.chromium.org/blink/trunk@179547 bbb929c8-8fbe-4397-9dbb-9b2b20218538