Refactor BlendMode into a separate class.

This moves the BlendMode code out of RenderLayer and into RenderLayerBlendInfo.

BUG=260898

Review URL: https://codereview.chromium.org/99883002

git-svn-id: svn://svn.chromium.org/blink/trunk@164790 bbb929c8-8fbe-4397-9dbb-9b2b20218538