[CSSRegions] Wrong auto-height region computation for nested named flows

In the case of an (inner) auto-height region that is absolutely positioned in a named flow that is displayed in an (outer) auto-height region,
when we are doing the layout for the named flow we need to add a "forced region break" also for the case of simplified layout, otherwise
the outer auto-height region will not get the chance to compute its auto-height, ending up with a huge value that was used to initialize
the auto-height computed value before the start of the layout algorithm.

BUG=269290

Review URL: https://chromiumcodereview.appspot.com/22544002

git-svn-id: svn://svn.chromium.org/blink/trunk@155992 bbb929c8-8fbe-4397-9dbb-9b2b20218538