Add DesktopRectTest for UnionWith() function

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-Original-Commit-Position: refs/heads/master@{#18201}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8eef7aa7cf165401fe4083b02334890097e5b003
2 files changed