Implement SkImageFilter support for feDropShadow. Add some new test cases to the reference filter tests, and
add some missing color-interpolation-filters="sRGB".

BUG=323639
R=fmalita

Review URL: https://codereview.chromium.org/100813003

git-svn-id: svn://svn.chromium.org/blink/trunk@163102 bbb929c8-8fbe-4397-9dbb-9b2b20218538