Remove CWV class and move setting User Agent to CWVWebViewConfiguration.

ios/web_view/internal/web_view_browser_state.mm

Index: ios/web_view/internal/web_view_browser_state.mm
diff --git a/ios/web_view/internal/web_view_browser_state.mm b/ios/web_view/internal/web_view_browser_state.mm
index 6a34819d4e913eeadac5e6246a68bd9e6412c824..c41432f649a98f1fb99b76df3609c1bc2da201ee 100644
--- a/ios/web_view/internal/web_view_browser_state.mm
+++ b/ios/web_view/internal/web_view_browser_state.mm
@@ -34,6 +34,11 @@ namespace ios_web_view {
 
 WebViewBrowserState::WebViewBrowserState(bool off_the_record)
     : web::BrowserState(), off_the_record_(off_the_record) {
+  // IO access is required to setup the browser state. In Chrome, this is
+  // already allowed during thread startup. However, startup time of
+  // ChromeWebView is not predetermined, so IO access is temporarily allowed.
+  bool wasIOAllowed = base::ThreadRestrictions::SetIOAllowed(true);
+
   CHECK(PathService::Get(base::DIR_APP_DATA, &path_));
 
   request_context_getter_ = new WebViewURLRequestContextGetter(
@@ -56,6 +61,8 @@ WebViewBrowserState::WebViewBrowserState(bool off_the_record)
   PrefServiceFactory factory;
   factory.set_user_prefs(user_pref_store);
   prefs_ = factory.Create(pref_registry.get());
+
+  base::ThreadRestrictions::SetIOAllowed(wasIOAllowed);
 }
 
 WebViewBrowserState::~WebViewBrowserState() = default;