DesktopRect::UnionWith() function has been added by change
https://codereview.webrtc.org/2845213002. This change adds test cases to cover
the newly added logic. More specifically, union between an empty rectangle and a
non-empty one or two empty rectangles.
BUG=webrtc:7541
Review-Url: https://codereview.webrtc.org/2891593003
Cr-Commit-Position: refs/heads/master@{#18201}
