Remove CWV class and move setting User Agent to CWVWebViewConfiguration.

ios/web_view/internal/cwv_web_view_configuration_internal.h

Index: ios/web_view/internal/cwv_web_view_configuration_internal.h
diff --git a/ios/web_view/internal/cwv_web_view_configuration_internal.h b/ios/web_view/internal/cwv_web_view_configuration_internal.h
index d7942d61ee69d2a58dd69632dd92476972ec11ad..87d9442992fc4123ebf4bd0e9204466b29b7a4a9 100644
--- a/ios/web_view/internal/cwv_web_view_configuration_internal.h
+++ b/ios/web_view/internal/cwv_web_view_configuration_internal.h
@@ -9,6 +9,7 @@
 
 namespace ios_web_view {
 class WebViewBrowserState;
+class WebViewWebClient;
 }  // namespace ios_web_view
 
 @interface CWVWebViewConfiguration ()
@@ -17,6 +18,9 @@ class WebViewBrowserState;
 @property(nonatomic, readonly, nonnull)
     ios_web_view::WebViewBrowserState* browserState;
 
+// Returns the web client, setting up the WebMainDelegate and WebMain if needed.
++ (nonnull ios_web_view::WebViewWebClient*)webClient;
+
 @end
 
 #endif  // IOS_WEB_VIEW_INTERNAL_CWV_WEB_VIEW_CONFIGURATION_INTERNAL_H_