Revert of Replace requirePageReconstruction with setCustomUserAgent

ios/web/web_state/ui/crw_web_controller.mm

Index: ios/web/web_state/ui/crw_web_controller.mm
diff --git a/ios/web/web_state/ui/crw_web_controller.mm b/ios/web/web_state/ui/crw_web_controller.mm
index 1f186d549373f07690ce3f2293f29be3e1d858cd..4d87b2c3da8df85af85d482cbd6a74a833044682 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -459,10 +459,8 @@
 // Returns YES if the user interacted with the page recently.
 @property(nonatomic, readonly) BOOL userClickedRecently;
 
-// User agent type of the transient item if any, the pending item if a
-// navigation is in progress or the last committed item otherwise.
-// Returns MOBILE, the default type, if navigation manager is nullptr or empty.
-@property(nonatomic, readonly) web::UserAgentType userAgentType;
+// Whether or not desktop user agent is used for the currentItem.
+@property(nonatomic, readonly) BOOL usesDesktopUserAgent;
 
 // Facade for Mojo API.
 @property(nonatomic, readonly) web::MojoFacade* mojoFacade;
@@ -479,18 +477,10 @@
 // unless the request was a POST.
 @property(nonatomic, readonly) NSDictionary* currentHTTPHeaders;
 
-// Updates web view's user agent according to |userAgentType|. It is no-op if
-// |userAgentType| is NONE.
-- (void)updateWebViewUserAgentFromUserAgentType:
-    (web::UserAgentType)userAgentType;
-
-// Requires that the next load rebuild the web view. This is expensive, and
-// should be used only in the case where something has changed that web view
-// only checks on creation, such that the whole object needs to be rebuilt.
-// TODO(stuartmorgan): Merge this and reinitializeWebViewAndReload:. They are
-// currently subtly different in terms of implementation, but are for
-// fundamentally the same purpose.
-- (void)requirePageReconstruction;
+// Requires page reconstruction if |item| has a non-NONE UserAgentType and it
+// differs from that of |fromItem|.
+- (void)updateDesktopUserAgentForItem:(web::NavigationItem*)item
+                previousUserAgentType:(web::UserAgentType)userAgentType;
 
 // Removes the container view from the hierarchy and resets the ivar.
 - (void)resetContainerView;
@@ -1626,13 +1616,6 @@
 
   [self ensureWebViewCreated];
 
-  // Update web view's user agent is called for every load, which may have
-  // performance implications because update web view's user agent may
-  // potentially send a message to a separate thread. However, this is not an
-  // issue for WKWebView because WKWebView's |setCustomUserAgent| is non-op if
-  // user agent stays thesame.
-  [self updateWebViewUserAgentFromUserAgentType:self.userAgentType];
-
   [self loadRequestForCurrentNavigationItem];
 }
 
@@ -2043,11 +2026,15 @@
 
   // Update the user agent before attempting the navigation.
   web::NavigationItem* toItem = items[index].get();
-  [self updateWebViewUserAgentFromUserAgentType:toItem->GetUserAgentType()];
-
-  web::NavigationItem* fromItem = sessionController.currentItem;
+  web::NavigationItem* previousItem = sessionController.currentItem;
+  web::UserAgentType previousUserAgentType =
+      previousItem ? previousItem->GetUserAgentType()
+                   : web::UserAgentType::NONE;
+  [self updateDesktopUserAgentForItem:toItem
+                previousUserAgentType:previousUserAgentType];
+
   BOOL sameDocumentNavigation =
-      [sessionController isSameDocumentNavigationBetweenItem:fromItem
+      [sessionController isSameDocumentNavigationBetweenItem:previousItem
                                                      andItem:toItem];
   if (sameDocumentNavigation) {
     [sessionController goToItemAtIndex:index];
@@ -2202,9 +2189,9 @@
   return rendererInitiatedWithoutInteraction || noNavigationItems;
 }
 
-- (web::UserAgentType)userAgentType {
+- (BOOL)usesDesktopUserAgent {
   web::NavigationItem* item = self.currentNavItem;
-  return item ? item->GetUserAgentType() : web::UserAgentType::MOBILE;
+  return item && item->GetUserAgentType() == web::UserAgentType::DESKTOP;
 }
 
 - (web::MojoFacade*)mojoFacade {
@@ -2243,14 +2230,15 @@
   return _passKitDownloader.get();
 }
 
-- (void)updateWebViewUserAgentFromUserAgentType:
-    (web::UserAgentType)userAgentType {
-  if (userAgentType == web::UserAgentType::NONE)
-    return;
-
-  NSString* userAgent =
-      base::SysUTF8ToNSString(web::GetWebClient()->GetUserAgent(userAgentType));
-  [_webView setCustomUserAgent:userAgent];
+- (void)updateDesktopUserAgentForItem:(web::NavigationItem*)item
+                previousUserAgentType:(web::UserAgentType)userAgentType {
+  if (!item)
+    return;
+  web::UserAgentType itemUserAgentType = item->GetUserAgentType();
+  if (itemUserAgentType == web::UserAgentType::NONE)
+    return;
+  if (itemUserAgentType != userAgentType)
+    [self requirePageReconstruction];
 }
 
 #pragma mark -
@@ -4052,7 +4040,7 @@
   // delegate must be specified.
   return web::BuildWKWebView(CGRectZero, config,
                              self.webStateImpl->GetBrowserState(),
-                             self.userAgentType);
+                             self.usesDesktopUserAgent);
 }
 
 - (void)setWebView:(WKWebView*)webView {