[CSS Blending] Implement the isolation property for SVG.

This patch adds the isolation functionality to SVG, performs
some cleanup on the current blend modes SVG implementation
and adds tests to validate the feature.

BUG=229615

Review URL: https://codereview.chromium.org/108163004

git-svn-id: svn://svn.chromium.org/blink/trunk@164028 bbb929c8-8fbe-4397-9dbb-9b2b20218538