First submission for support of blending in SVG + added a number of ref tests that show all blend modes and test conditions for isolation.

The CSS property is already parsed and stored (mix-blend-mode) and the blend mode themselves are already implemented in skia.
Launch bug is http://crbug.com/229615

BUG=260927

Review URL: https://chromiumcodereview.appspot.com/17115010

git-svn-id: svn://svn.chromium.org/blink/trunk@154607 bbb929c8-8fbe-4397-9dbb-9b2b20218538