Enable deferred image filters by default.

This change switches SVG-on-SVG filters over to the deferred 
filter path. Flip the flag in Blink first, so we can
rebaseline the tests at the same time. If things go pear-
shaped, we'll revert here in Blink. If not, a Chrome 
change will follow to flip the command-line flag from 
--enable-deferred-filters to --disable-deferred-filters, 
and then a final Blink patch will remove the override in 
WebPreferences added here.

R=fmalita
BUG=368265

Review URL: https://codereview.chromium.org/258793007

git-svn-id: svn://svn.chromium.org/blink/trunk@173058 bbb929c8-8fbe-4397-9dbb-9b2b20218538