[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..509fc94f1d3e6b94b3556f8d546fbd3cf18651fb 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',
@@ -365,6 +364,7 @@
     },
     # Target shared by ios_web and CrNet.
     {
+      # GN version: //ios/web:core
       'target_name': 'ios_web_core',
       'type': 'static_library',
       'dependencies': [
@@ -381,6 +381,7 @@
       ],
     },
     {
+      # GN version: //ios/web:web_bundle_ui
       'target_name': 'ios_web_js_bundle_ui',
       'type': 'none',
       'variables': {
@@ -411,6 +412,7 @@
       ],
     },
     {
+      # GN version: //ios/web:web_bundle_wk
       'target_name': 'ios_web_js_bundle_wk',
       'type': 'none',
       'variables': {
@@ -441,6 +443,7 @@
       ],
     },
     {
+      # GN version: //ios/web:js_resources
       'target_name': 'js_resources',
       'type': 'none',
       'dependencies': [
@@ -464,6 +467,7 @@
       ],
     },
     {
+      # GN version: //ios/web:test_support
       'target_name': 'test_support_ios_web',
       'type': 'static_library',
       'dependencies': [
@@ -550,6 +554,7 @@
       ],
     },
     {
+      # GN version: //ios/web:user_agent
       'target_name': 'user_agent',
       'type': 'static_library',
       'include_dirs': [