Ensure logical height is overridden for stretch-aligned flex-box children

With an SVG in a flex-box context (as a flex-item), it's possible for a
document-wide layout to clear the override-height, and then issue a
partial (<svg> as layout-root) layout that recomputes the logical height
to whatever the <svg> root thinks it should be - disregarding the flex-box
context.
To fix this, make sure that a stretch-aligned flex-item always has an
override-height set, even if the logical height appear to be up-to-date.

BUG=397085

Review URL: https://codereview.chromium.org/423133002

git-svn-id: svn://svn.chromium.org/blink/trunk@183293 bbb929c8-8fbe-4397-9dbb-9b2b20218538