Add a data_dep for osmesa to //ui/gl and run ui_unittests.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
 
 # This target will be built if no target is specified when invoking ninja.
 group("default") {
   testonly = true
 
   deps = [
     "//apps",
     "//base:base_unittests",
     "//benchmarks",
     "//crypto:crypto_unittests",
     "//examples",
     "//mojo",
     "//mojom",
     "//services",
     "//shell",
   ]
 
   if (is_linux) {
-    deps += [ "//fusl" ]
+    deps += [
+      "//fusl",
+
+      # We need this to run tests (especially on bots).
+      "//third_party/mesa:osmesa",
+    ]
   }
 
   # TODO(cstout): fix sandbox build for fnl/musl
   if (is_linux && !is_fnl) {
     deps += [
       "//sandbox/linux:sandbox",
       "//sandbox/linux:sandbox_linux_unittests",
     ]
   }
   if (is_linux && !use_ozone) {
@@ skipping 23 matching lines @@
 if (use_ozone) {
   # This isn't in the default target as it's a bit wonky right now.
   group("ozone_tests") {
     testonly = true
     deps = [
       "//ui/ozone:ozone_unittests",
       "//ui/ozone/demo",
     ]
   }
 }