[CSS Blending] Fix blending when mix-blend-mode is set from javascript

If mix-blend-mode is set from javascript to an element that is also a
stacking context, blending is not limited to the parent stacking
context.

BUG=229615

Review URL: https://codereview.chromium.org/90983002

git-svn-id: svn://svn.chromium.org/blink/trunk@162763 bbb929c8-8fbe-4397-9dbb-9b2b20218538