Also, fixes the stocks list to work properly including hit testing at
the bottom of the screen.

New classes:

RenderViewport: a class that supports positioning a child inside
itself and offsetting it.

Viewport: a RenderObjectWrapper that wraps RenderViewport.

ScrollableViewport: a Component that hooks Viewport up to some
scrolling behaviour.

Code changes:

RenderBlock now only works when it has an unbounded height constraint.
I removed the clipping in there since it's no longer needed.

I made FixedHeightScrollable use Viewport instead of hand-rolling its
clipping with Transform and Clip. This is what fixes the stocks list
hit testing at the bottom of the screen.

I made anywhere that used to use Block now use ScrollableViewport.

RenderFlex now takes a list of children.

Justifications for test changes:

tests/examples/stocks: changing FixedHeightScrollable to use a
RenderViewport instead of a RenderClipRect/RenderTransform combination
removes the use of an actual transform.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1223153004 .