Add CWVUserContentController which enables injecting JavaScripts.

ios/web_view/internal/cwv_user_script.mm

Index: ios/web_view/internal/cwv_user_script.mm
diff --git a/ios/web_view/internal/cwv_user_script.mm b/ios/web_view/internal/cwv_user_script.mm
new file mode 100644
index 0000000000000000000000000000000000000000..4e11666975cba4fde96af28c0f8b2e9c92da7ca1
--- /dev/null
+++ b/ios/web_view/internal/cwv_user_script.mm
@@ -0,0 +1,23 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import "ios/web_view/public/cwv_user_script.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+@implementation CWVUserScript
+
+@synthesize source = _source;
+
+- (nullable instancetype)initWithSource:(nonnull NSString*)source {
+  self = [super init];
+  if (self) {
+    _source = [source copy];
+  }
+  return self;
+}
+
+@end