Adjust error-handling for invalid filter primitive references

When a filter primitive has an 'in' (or, where applicable, 'in2') attribute
that references a non-existent result, treat that reference as having the
default value (==no value) instead of producing an error.
This aligns with Presto and IE.

BUG=411833

Review URL: https://codereview.chromium.org/558693002

git-svn-id: svn://svn.chromium.org/blink/trunk@181733 bbb929c8-8fbe-4397-9dbb-9b2b20218538