Compile screen capturer mac test only when is_mac=true.

Starting from [1], GN is not using the global filter to implicitly
compile sources only on some platforms.

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/2426564

Bug: None
Change-Id: I6f547e0bddc9cc87625d5b40a5aa1148b8dfd2d2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185808
Reviewed-by: Andrey Logvin <landrey@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32210}