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 4d87b2c3da8df85af85d482cbd6a74a833044682..1f186d549373f07690ce3f2293f29be3e1d858cd 100644
--- a/ios/web/web_state/ui/crw_web_controller.mm
+++ b/ios/web/web_state/ui/crw_web_controller.mm
@@ -459,8 +459,10 @@ @interface CRWWebController ()<CRWContextMenuDelegate,
 // Returns YES if the user interacted with the page recently.
 @property(nonatomic, readonly) BOOL userClickedRecently;
 
-// Whether or not desktop user agent is used for the currentItem.
-@property(nonatomic, readonly) BOOL usesDesktopUserAgent;
+// 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;
 
 // Facade for Mojo API.
 @property(nonatomic, readonly) web::MojoFacade* mojoFacade;
@@ -477,10 +479,18 @@ @interface CRWWebController ()<CRWContextMenuDelegate,
 // unless the request was a POST.
 @property(nonatomic, readonly) NSDictionary* currentHTTPHeaders;
 
-// 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;
+// 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;
 
 // Removes the container view from the hierarchy and resets the ivar.
 - (void)resetContainerView;
@@ -1616,6 +1626,13 @@ - (void)loadCurrentURLInWebView {
 
   [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];
 }
 
@@ -2026,15 +2043,11 @@ - (void)goToItemAtIndex:(int)index {
 
   // Update the user agent before attempting the navigation.
   web::NavigationItem* toItem = items[index].get();
-  web::NavigationItem* previousItem = sessionController.currentItem;
-  web::UserAgentType previousUserAgentType =
-      previousItem ? previousItem->GetUserAgentType()
-                   : web::UserAgentType::NONE;
-  [self updateDesktopUserAgentForItem:toItem
-                previousUserAgentType:previousUserAgentType];
+  [self updateWebViewUserAgentFromUserAgentType:toItem->GetUserAgentType()];
 
+  web::NavigationItem* fromItem = sessionController.currentItem;
   BOOL sameDocumentNavigation =
-      [sessionController isSameDocumentNavigationBetweenItem:previousItem
+      [sessionController isSameDocumentNavigationBetweenItem:fromItem
                                                      andItem:toItem];
   if (sameDocumentNavigation) {
     [sessionController goToItemAtIndex:index];
@@ -2189,9 +2202,9 @@ - (BOOL)shouldClosePageOnNativeApplicationLoad {
   return rendererInitiatedWithoutInteraction || noNavigationItems;
 }
 
-- (BOOL)usesDesktopUserAgent {
+- (web::UserAgentType)userAgentType {
   web::NavigationItem* item = self.currentNavItem;
-  return item && item->GetUserAgentType() == web::UserAgentType::DESKTOP;
+  return item ? item->GetUserAgentType() : web::UserAgentType::MOBILE;
 }
 
 - (web::MojoFacade*)mojoFacade {
@@ -2230,15 +2243,14 @@ - (CRWPassKitDownloader*)passKitDownloader {
   return _passKitDownloader.get();
 }
 
-- (void)updateDesktopUserAgentForItem:(web::NavigationItem*)item
-                previousUserAgentType:(web::UserAgentType)userAgentType {
-  if (!item)
+- (void)updateWebViewUserAgentFromUserAgentType:
+    (web::UserAgentType)userAgentType {
+  if (userAgentType == web::UserAgentType::NONE)
     return;
-  web::UserAgentType itemUserAgentType = item->GetUserAgentType();
-  if (itemUserAgentType == web::UserAgentType::NONE)
-    return;
-  if (itemUserAgentType != userAgentType)
-    [self requirePageReconstruction];
+
+  NSString* userAgent =
+      base::SysUTF8ToNSString(web::GetWebClient()->GetUserAgent(userAgentType));
+  [_webView setCustomUserAgent:userAgent];
 }
 
 #pragma mark -
@@ -4040,7 +4052,7 @@ - (WKWebView*)webViewWithConfiguration:(WKWebViewConfiguration*)config {
   // delegate must be specified.
   return web::BuildWKWebView(CGRectZero, config,
                              self.webStateImpl->GetBrowserState(),
-                             self.usesDesktopUserAgent);
+                             self.userAgentType);
 }
 
 - (void)setWebView:(WKWebView*)webView {