Remove some validation and asserts from tessellating path renderer.

Remove some validation and asserts from tessellating path renderer.

In some cases, resolving an intersection can cause a vertex to go
slightly out-of-order with edges which have already been processed.
This doesn't cause any algorithmic errors, but it's difficult to detect
without impacting performance significantly.

Also, the GPU infrastructure fires asserts when attempting
to allocate 0-length vertex buffers. Early-out instead,
since there's nothing to draw.

Review URL: https://codereview.chromium.org/1032253005