Reland: Run gl_unittests with Ozone X11.

Add a new test target gl_unittests_ozonex that runs with
--ozone-platform=x11. The Ozone CrOS bot will now run gl_unittests with
both Ozone headless and Ozone X11.

The test requires an X server to be running on the bot. The mb.py script
is modified to look for GN arg use_ozone=true and runtime flag
--ozone-platform=x11. In this case, it runs the test using xvfb.

Original commit crrev.com/2493063003 was reverted due to
crbug.com/665895. There was no dependency on xdisplaycheck by the new
test target that needs it. This caused problems during the isolate step
when xdisplaycheck wasn't found.

BUG=664570

Review-Url: https://codereview.chromium.org/2509033006
Cr-Commit-Position: refs/heads/master@{#435000}
diff --git a/BUILD.gn b/BUILD.gn
index b453d53f..f06d672 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -18,6 +18,7 @@
 import("//remoting/remoting_enable.gni")
 import("//third_party/openh264/openh264_args.gni")
 import("//tools/ipc_fuzzer/ipc_fuzzer.gni")
+import("//ui/ozone/ozone.gni")
 import("//v8/gni/v8.gni")
 import("//v8/snapshot_toolchain.gni")
 
@@ -373,7 +374,7 @@
     ]
   }
 
-  if (use_x11) {
+  if (use_x11 || ozone_platform_x11) {
     deps += [ "//tools/xdisplaycheck" ]
   }