Add CWVUserContentController which enables injecting JavaScripts.

ios/web_view/internal/cwv_web_view_configuration.mm

Index: ios/web_view/internal/cwv_web_view_configuration.mm
diff --git a/ios/web_view/internal/cwv_web_view_configuration.mm b/ios/web_view/internal/cwv_web_view_configuration.mm
index f29c00200f7f99c04fed224227d111cba1ff0edb..d8916d719da520424576cd898b40c9e11e064559 100644
--- a/ios/web_view/internal/cwv_web_view_configuration.mm
+++ b/ios/web_view/internal/cwv_web_view_configuration.mm
@@ -5,6 +5,7 @@
 #import "ios/web_view/public/cwv_web_view_configuration.h"
 #import "ios/web_view/internal/cwv_web_view_configuration_internal.h"
 
+#import "ios/web_view/internal/cwv_user_content_controller_internal.h"
 #import "ios/web_view/internal/web_view_browser_state.h"
 #import "ios/web_view/internal/web_view_web_client.h"
 
@@ -23,6 +24,8 @@
   ios_web_view::WebViewBrowserState* _browserState;
 }
 
+@synthesize userContentController = _userContentController;
+
 + (instancetype)defaultConfiguration {
   static dispatch_once_t once;
   static CWVWebViewConfiguration* configuration;
@@ -51,6 +54,8 @@
   self = [super init];
   if (self) {
     _browserState = browserState;
+    _userContentController =
+        [[CWVUserContentController alloc] initWithConfiguration:self];
   }
   return self;
 }