[iOS][GN] Port ios_web_unittests to build with gn

ios/web/ios_web.gyp

Index: ios/web/ios_web.gyp
diff --git a/ios/web/ios_web.gyp b/ios/web/ios_web.gyp
index a1ab0fff1dba90a6217265b58725e8ac84e606ae..ce01f7331948a1b209f2bb42b9f11643566a75a3 100644
--- a/ios/web/ios_web.gyp
+++ b/ios/web/ios_web.gyp
@@ -41,6 +41,7 @@
     # This will become unnecessary once Chrome switches to using ios_web_thread,
     # at which point that will be folded into this target.
     {
+      # GN version: //ios/web
       'target_name': 'ios_web',
       'type': 'static_library',
       'include_dirs': [
@@ -174,8 +175,6 @@
         'public/string_util.h',
         'public/url_scheme_util.h',
         'public/url_util.h',
-        'public/user_agent.h',
-        'public/user_agent.mm',
         'public/user_metrics.h',
         'public/web/url_data_source_ios.h',
         'public/web_client.h',
@@ -320,6 +319,8 @@
         'webui/web_ui_ios_impl.mm',
       ],
       'link_settings': {
+        # TODO(eugenebut): change to regular linking once support for iOS 7
+        # is dropped. http://crbug.com/541549
         'xcode_settings': {
           'OTHER_LDFLAGS': [
             '-weak_framework WebKit',
@@ -365,6 +366,7 @@
     },
     # Target shared by ios_web and CrNet.
     {
+      # GN version: //ios/web:core
       'target_name': 'ios_web_core',
       'type': 'static_library',
       'dependencies': [
@@ -381,6 +383,7 @@
       ],
     },
     {
+      # GN version: //ios/web:web_bundle_ui
       'target_name': 'ios_web_js_bundle_ui',
       'type': 'none',
       'variables': {
@@ -411,6 +414,7 @@
       ],
     },
     {
+      # GN version: //ios/web:web_bundle_wk
       'target_name': 'ios_web_js_bundle_wk',
       'type': 'none',
       'variables': {
@@ -441,6 +445,7 @@
       ],
     },
     {
+      # GN version: //ios/web:js_resources
       'target_name': 'js_resources',
       'type': 'none',
       'dependencies': [
@@ -464,6 +469,7 @@
       ],
     },
     {
+      # GN version: //ios/web:test_support
       'target_name': 'test_support_ios_web',
       'type': 'static_library',
       'dependencies': [
@@ -550,6 +556,7 @@
       ],
     },
     {
+      # GN version: //ios/web:user_agent
       'target_name': 'user_agent',
       'type': 'static_library',
       'include_dirs': [