Check for use_x11 before runnig desktop_capture_modules_tests on linux.

webrtc/webrtc.gni

Index: webrtc/webrtc.gni
diff --git a/webrtc/webrtc.gni b/webrtc/webrtc.gni
index 4c34eae1922a0535f36b6ef5cb0f3ceac2d8490a..e6d746a49ea0df20adab7850a18ee875b7fe0afe 100644
--- a/webrtc/webrtc.gni
+++ b/webrtc/webrtc.gni
@@ -10,6 +10,7 @@ import("//build/config/arm.gni")
 import("//build/config/features.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
+import("//build/config/ui.gni")
 import("//build_overrides/build.gni")
 import("//testing/test.gni")
 
@@ -172,7 +173,7 @@ rtc_libyuv_dir = "//third_party/libyuv"
 rtc_opus_dir = "//third_party/opus"
 
 # Desktop capturer is supported only on Windows, OSX and Linux.
-rtc_desktop_capture_supported = is_win || is_mac || is_linux
+rtc_desktop_capture_supported = is_win || is_mac || (is_linux && use_x11)
 
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 # Templates