Remove unneeded flow thread awareness from accumulateOffsetTowardsAncestor().

We no longer have out-of-flow flow threads. Those were only used by CSS regions.

R=jchaffraix@chromium.org

Review URL: https://codereview.chromium.org/616453002

git-svn-id: svn://svn.chromium.org/blink/trunk@183313 bbb929c8-8fbe-4397-9dbb-9b2b20218538