content: Port layout_test_helper target to GN build.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index d41f2cdfc418ec5fb800200aae2ccc31ba95df63..6166a8e6fe044c3ae342078d7e7a22e453c3d04a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -752,8 +752,8 @@ if (!is_ios) {
 
     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)
       ]
     }
 
@@ -764,7 +764,7 @@ if (!is_ios) {
     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",
       ]
     }