Move all filters-related files from core/platform to platform.

Also lots of pre-submit test-related cleanup.

R=abarth@chromium.org,senorblanco@chromium.org
BUG=303332

Review URL: https://codereview.chromium.org/107713004

git-svn-id: svn://svn.chromium.org/blink/trunk@163548 bbb929c8-8fbe-4397-9dbb-9b2b20218538