Fix DesktopSize::is_empty() for the case when only width or only height is 0.
Fix DesktopSize::is_empty() for the case when only width or only height is 0.

BUG=crbug.com/358909
R=wez@chromium.org

Review URL: https://webrtc-codereview.appspot.com/11479004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 74f6074ec15f808482739c615f80243d0f32bef7

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