-Make display:flex, flex-direction: column, flex-shrink: 1 the default.
-Simplify StyleAdjuster::adjustStyleForAlignment to remove special cases we
won't need as we make flex the default and remove absolute positioning.
-Fix a bug this exposed in column flexboxes where we'd apply the wrong edge
of border/padding/margin.
-For now leave the default of align-items:stretch. The main change here is
that iframe/img will do width:auto the same as blocks (i.e. the width of
the parent). I think this is a good change, but we'll have to see how it feels
in practice.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1061163002