Mark gtest and gmock as testonly in GN.

ui/base/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")
 
 if (is_android) {
   import("//build/config/android/config.gni")
   import("//build/config/android/rules.gni")
 }
@@ skipping 534 matching lines @@
   }
 
   if (is_android || is_ios) {
     sources -= [
       "device_form_factor_desktop.cc",
     ]
   }
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "test/ui_controls.h",
     "test/ui_controls_aura.cc",
     "test/ui_controls_internal_win.cc",
     "test/ui_controls_internal_win.h",
     "test/ui_controls_mac.mm",
     "test/ui_controls_win.cc",
   ]
 
   deps = [
@@ skipping 24 matching lines @@
       "../android/java/src/org/chromium/ui/base/LocalizationUtils.java",
       "../android/java/src/org/chromium/ui/base/ResourceBundle.java",
       "../android/java/src/org/chromium/ui/base/SelectFileDialog.java",
       "../android/java/src/org/chromium/ui/base/TouchDevice.java",
       "../android/java/src/org/chromium/ui/base/ViewAndroid.java",
       "../android/java/src/org/chromium/ui/base/WindowAndroid.java",
     ]
     jni_package = "base"
   }
 }