Merge branch 'crossOverlays-for-18.09' (early part) into tempory branch

This was an early "pre-release" to stable, with a warning that the
interface is not stable yet (thought we don't actually expect it to
change).

We do expect to make many static build fixes, however, which should go
to master and staging, so we merge the cherry-picks back into master so
we can do those fixes against:

  git merge-base upstream/master upstream/release-18.09

and merge into both master and release-18.09. This is easier than
cherry-picking when one knows in advance they are targeting multiple
branches.

No files should be changed with this merge.