Re-land "Replace requirePageReconstruction with setCustomUserAgent"

ios/web/web_state/web_view_internal_creation_util.mm

Index: ios/web/web_state/web_view_internal_creation_util.mm
diff --git a/ios/web/web_state/web_view_internal_creation_util.mm b/ios/web/web_state/web_view_internal_creation_util.mm
index c13b8b77713499d7c5a241e873d4340c49f51173..5095110ff5225cd65dde288949e61442d8614181 100644
--- a/ios/web/web_state/web_view_internal_creation_util.mm
+++ b/ios/web/web_state/web_view_internal_creation_util.mm
@@ -36,7 +36,7 @@ void VerifyWKWebViewCreationPreConditions(
 WKWebView* BuildWKWebView(CGRect frame,
                           WKWebViewConfiguration* configuration,
                           BrowserState* browser_state,
-                          BOOL use_desktop_user_agent,
+                          UserAgentType user_agent_type,
                           id<CRWContextMenuDelegate> context_menu_delegate) {
   VerifyWKWebViewCreationPreConditions(browser_state, configuration);
 
@@ -44,11 +44,10 @@ void VerifyWKWebViewCreationPreConditions(
   WKWebView* web_view =
       [[WKWebView alloc] initWithFrame:frame configuration:configuration];
 
-  // Set the user agent.
-  UserAgentType user_agent_type =
-      use_desktop_user_agent ? UserAgentType::DESKTOP : UserAgentType::MOBILE;
-  web_view.customUserAgent = base::SysUTF8ToNSString(
-      web::GetWebClient()->GetUserAgent(user_agent_type));
+  if (user_agent_type != web::UserAgentType::NONE) {
+    web_view.customUserAgent = base::SysUTF8ToNSString(
+        web::GetWebClient()->GetUserAgent(user_agent_type));
+  }
 
   // By default the web view uses a very sluggish scroll speed. Set it to a more
   // reasonable value.
@@ -77,17 +76,16 @@ void VerifyWKWebViewCreationPreConditions(
 WKWebView* BuildWKWebView(CGRect frame,
                           WKWebViewConfiguration* configuration,
                           BrowserState* browser_state,
-                          BOOL use_desktop_user_agent) {
-  return BuildWKWebView(frame, configuration, browser_state,
-                        use_desktop_user_agent, nil);
+                          UserAgentType user_agent_type) {
+  return BuildWKWebView(frame, configuration, browser_state, user_agent_type,
+                        nil);
 }
 
 WKWebView* BuildWKWebView(CGRect frame,
                           WKWebViewConfiguration* configuration,
                           BrowserState* browser_state) {
-  BOOL use_desktop_user_agent = NO;
   return BuildWKWebView(frame, configuration, browser_state,
-                        use_desktop_user_agent);
+                        UserAgentType::MOBILE);
 }
 
 }  // namespace web