[escher] Unbreak Escher waterfall demo.

Relax an overzealous DCHECK in the rounded-rect tessellator which
was exploding when provided with more memory than required.

It's unclear what started to trigger this, because no Escher code
has changed since the last time the Waterfall demo ran successfully.

TEST=N/A (existing Scenic code was already not triggering this
condition)

Change-Id: I05ea520b59ac973680ac0ff04d64b9a5abad36be
1 file changed