Remove the use of overrideLogicalContentHeight from the regions'
processing model. This prevents flexbox containers from overriding
the height value computed when flowing content into auto-height regions.

Diff from the initial CL: fix initialization in the constructor.
Initially reviewed here: https://codereview.chromium.org/18319004/

BUG=256233

Review URL: https://chromiumcodereview.appspot.com/20733002

git-svn-id: svn://svn.chromium.org/blink/trunk@155119 bbb929c8-8fbe-4397-9dbb-9b2b20218538