Use treescope for filter id references

This patch fixes a bug where ReferenceFilterBuilder used the document
to lookup id references instead of the element's treescope. This fixes
filters that appear in shadow trees and is the last document-based
getElementId call in the SVG codebase.

BUG=364358

Review URL: https://codereview.chromium.org/240333008

git-svn-id: svn://svn.chromium.org/blink/trunk@171909 bbb929c8-8fbe-4397-9dbb-9b2b20218538