Update Ozone documentation for Linux Desktop

Currently Chrome/Ozone/mash does not run anymore on Linux Desktop.
This CL updates the documentation to point to Igalia's development
branch for Ozone Wayland [1] where issues are being addressed.
The `--mash` and `--window-manager` command line switches are also
replaced with the `--mus` switch introduced in [2].

[1] https://github.com/Igalia/chromium/tree/ozone-wayland-dev
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=704985

Review-Url: https://codereview.chromium.org/2797133005
Cr-Commit-Position: refs/heads/master@{#462144}
1 file changed