content: Port layout_test_helper target to GN build.

BUILD.gn

 # Copyright (c) 2013 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.
 
 # This is the root build file for GN. GN will start processing by loading this
 # file, and recursively load all dependencies until all dependencies are either
 # resolved or known not to exist (which will cause the build to fail). So if
 # you add a new build file, there must be some path of dependencies from this
 # file to your new one or GN won't know about it.
 
@@ skipping 734 matching lines @@
         "//breakpad:dump_syms($host_toolchain)",
         "//breakpad:minidump_stackwalk($host_toolchain)",
         "//content/shell/android:content_shell_apk",
       ]
     } else {
       deps += [ "//content/shell:content_shell" ]
     }
 
     if (is_win) {
       deps += [
+        "//content/shell:layout_test_helper",
         #'../content/content_shell_and_tests.gyp:content_shell_crash_service', TODO(GYP)
-        #'../content/content_shell_and_tests.gyp:layout_test_helper', TODO(GYP)
       ]
     }
 
     if (!is_win && !is_android) {
       deps += [ "//breakpad:minidump_stackwalk" ]
     }
 
     if (is_mac) {
       deps += [
         "//breakpad:dump_syms($host_toolchain)",
-        #'../content/content_shell_and_tests.gyp:layout_test_helper', TODO(GYP)
+        "//content/shell:layout_test_helper",
       ]
     }
 
     if (is_linux) {
       deps += [ "//breakpad:dump_syms($host_toolchain)" ]
     }
   }
 }
 
 if (is_linux) {
@@ skipping 130 matching lines @@
       "//ui/views:views_unittests",  # TooltipControllerTest failures
       "//ui/wm:wm_unittests",  # PASSES 4/21/2015
       "//url:url_unittests",  # PASSES 4/17/2015
 
       # TODO(GYP) nacl_integration
       # TODO(GYP) telemetry_perf_unittests
       # TODO(GYP) telemetry_unittests
     ]
   }
 }