[CSS Blending] Fix isolation for the hardware path.
For "isolation:isolate" with accelerated elements, an ASSERT was
triggered in RenderLayerCompositor::subtreeReasonsForCompositing.
This was caused by errorneously including |hasIsolation| to RenderObject::CreateGroup.
Also, removing the comment from the method since it is misleading.

BUG=229615

Review URL: https://codereview.chromium.org/77723004

git-svn-id: svn://svn.chromium.org/blink/trunk@162360 bbb929c8-8fbe-4397-9dbb-9b2b20218538