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}

modules/desktop_capture/BUILD.gn

@@ -124,10 +124,12 @@ if (rtc_include_tests) {
     if (rtc_desktop_capture_supported) {
       sources += [
         "screen_capturer_helper_unittest.cc",
-        "screen_capturer_mac_unittest.cc",
         "screen_capturer_unittest.cc",
         "window_capturer_unittest.cc",
       ]
+      if (is_mac) {
+        sources += [ "screen_capturer_mac_unittest.cc" ]
+      }
       deps += [ ":desktop_capture_mock" ]
       public_configs = [ ":x11_config" ]
     }