[CSS Blending] SVG overlapping elements within a group don't blend together.

For a group of overlapping elements in an SVG, if mix-blend-mode is
specified on the group, the intersection of any overlapping elements also
incorrectly blends. This patch addresses the issue.
The problem is similar to https://codereview.chromium.org/104663003/

BUG=229615

Review URL: https://codereview.chromium.org/98543012

git-svn-id: svn://svn.chromium.org/blink/trunk@164504 bbb929c8-8fbe-4397-9dbb-9b2b20218538