GitLab

Mustash: Ensure surfaces submitted to Mus by WM and embedders contain Surfaces with embeded content.

- Use SurfaceLayers in embedders and Window Manager to properly position the
  embedded content in the layer tree.

- WM no longer uses underlay surfaces for window decorations. Instead they are
  submitted to Mus as part of the WM's surface.

- FrameGenerator now only creates SurfaceDrawQuad for the top-level window
  rather than recursively creating SurfaceDrawQuad for all windows in the
  window tree, since all surfaces are now part of the tree rooted at the
  surface of the top-level window.

BUG=672943,669964

Review-Url: https://codereview.chromium.org/2580063002
Cr-Commit-Position: refs/heads/master@{#441492}