GitLab

Resourceless software draw can only have a single root layer
render surface. Since decision to create render surface has
been moved to main thread, this condition no longer holds
and the mismatch causes a crash in LayerIterator.

Instead tweak the logic to only *use* the root layer render
surface, while being possible to to have more than one
render surface in the tree. This means only the root render
layer can be a render target.

BUG=466695

Review URL: https://codereview.chromium.org/1007623002

Cr-Commit-Position: refs/heads/master@{#320821}