gn: Add a missing test.

BUG=chromium:604060

Change-Id: I7e9b8c1bd7cb14daa15575cfcfe9ba3e8a1e6d1b
Reviewed-on: https://chromium-review.googlesource.com/344701
Tryjob-Request: Nico Weber <thakis@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

src/tests/BUILD.gn

@@ -64,6 +64,11 @@ if (is_win || is_linux || is_mac) {
                              ".",
                              "../..")
     }
+    if (use_x11) {
+      sources += rebase_path(end2end_gypi.angle_end2end_tests_x11_sources,
+                             ".",
+                             "../..")
+    }
 
     sources += [ "//gpu/angle_end2end_tests_main.cc" ]