Don't access compositing state for blending in RenderLayer add/remove child

Code to update the blending status in the RenderLayer hierarchy when a child is added or removed current invokes paintsWithBlendMode which calls compositingState() when it should not. This patch removes the compositing state check in this case, which means we may do more work.

All existing tests pass.

R=senorblanco@chromium.org
BUG=343759

Review URL: https://codereview.chromium.org/211103007

git-svn-id: svn://svn.chromium.org/blink/trunk@170220 bbb929c8-8fbe-4397-9dbb-9b2b20218538