Move ScreenCapturer 'real' tests out of screen_capturer_unittest.cc.

webrtc/modules/desktop_capture/BUILD.gn

Index: webrtc/modules/desktop_capture/BUILD.gn
diff --git a/webrtc/modules/desktop_capture/BUILD.gn b/webrtc/modules/desktop_capture/BUILD.gn
index 7b8638a5506141882d93e69faed53de1041eb1f4..04e9f90019d09d49fc6d179bbcbb1d6d5b60cf43 100644
--- a/webrtc/modules/desktop_capture/BUILD.gn
+++ b/webrtc/modules/desktop_capture/BUILD.gn
@@ -30,6 +30,26 @@ rtc_static_library("primitives") {
   ]
 }
 
+if (rtc_include_tests) {
+  source_set("desktop_capture_test_tools") {
+    testonly = true
+
+    public_deps = [
+      ":desktop_capture",
+    ]
+
+    sources = [
+      "rgba_color.cc",
+      "rgba_color.h",
+      "screen_drawer.cc",
+      "screen_drawer.h",
+      "screen_drawer_linux.cc",
+      "screen_drawer_mac.cc",
+      "screen_drawer_win.cc",
+    ]
+  }
+}
+
 rtc_static_library("desktop_capture") {
   sources = [
     "cropped_desktop_frame.cc",