Avoid blowing away the intrinsicContentLogicalHeight when stretching height in FlexBox

Cache and reset the intrinsic content logical height of children when stretching them
in FlexBox. This is to avoid them reseting it to the stretched height, which leads
us to improperly avoiding layout later when stretching is no longer necessary.

This solution is too fragile, but I couldn't come up with one that was more robust
but doesn't break the optimization.

BUG=409779

Review URL: https://codereview.chromium.org/715083002

git-svn-id: svn://svn.chromium.org/blink/trunk@185173 bbb929c8-8fbe-4397-9dbb-9b2b20218538