This patch make grid layout much more flexible. The behavior is factored
out into a GridDelegate that's modeled after the custom layout
delegates. The patch includes a MaxTileWidthGridDelegate that implements
the old behavior and a FixedColumnCountGridDelegate that implements a
grid layout with a fixed number of columns.

Fixes #1048

RenderView has to be a singleton for sanity during tests, otherwise they
all end up in the dirty lists and we end up pumping all of them each frame.

@abarth