Add gn implementation of //ios/web/shell:ios_web_shell.

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 169 matching lines @@
       "//ui/display:display_unittests",
       "//ui/events:events_unittests",
       "//ui/gl:gl_unittests",
       "//ui/touch_selection:ui_touch_selection_unittests",
     ]
   } else {
     deps += [
       "//ios/net:ios_net_unittests",
       "//ios/public/provider/web",
       "//ios/testing:ocmock_support_unittest",
+      "//ios/web/shell:ios_web_shell",
       "//ios/web:ios_web_unittests",
     ]
   }
 
   deps += root_extra_deps
 
   if (enable_extensions) {
     deps += [ "//extensions/shell:app_shell_unittests" ]
   }
 
@@ skipping 604 matching lines @@
       if (target_cpu == "x86") {
         deps += [
           # "//content/shell:crash_service_win64", TODO(GYP): crbug.com/537009
         ]
       }
     } else {
       deps += [ "//breakpad:minidump_stackwalk($host_toolchain)" ]
     }
   }
 }