Add CWVUserContentController which enables injecting JavaScripts.

ios/web_view/internal/cwv_user_script.mm

+// 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;
+
+- (instancetype)initWithSource:(NSString*)source {
+  self = [super init];
+  if (self) {
+    _source = source;

[Eugene But (OOO till 7-30), 2017/03/21 16:47:55]

_source = [source copy];

[Hiroshi Ichikawa, 2017/03/22 04:52:52]

Done.

+  }
+  return self;
+}
+
+@end