Add CWVUserContentController which enables injecting JavaScripts.

ios/web_view/internal/cwv_user_content_controller.mm

Index: ios/web_view/internal/cwv_user_content_controller.mm
diff --git a/ios/web_view/internal/cwv_user_content_controller.mm b/ios/web_view/internal/cwv_user_content_controller.mm
new file mode 100644
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+++ b/ios/web_view/internal/cwv_user_content_controller.mm
@@ -0,0 +1,62 @@
+// 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_content_controller.h"
+#import "ios/web_view/internal/cwv_user_content_controller_internal.h"
+
+#import "ios/web_view/internal/cwv_web_view_configuration_internal.h"
+#include "ios/web_view/internal/web_view_browser_state.h"
+#import "ios/web_view/internal/web_view_early_page_script_provider.h"
+#import "ios/web_view/public/cwv_user_script.h"
+
+@interface CWVUserContentController ()
+@property(weak, nonatomic) CWVWebViewConfiguration* configuration;
+@end
+
+@implementation CWVUserContentController {
+  NSMutableArray<CWVUserScript*>* _userScripts;
+}
+
+@synthesize configuration = _configuration;
+
+- (nonnull instancetype)initWithConfiguration:
+    (nonnull __weak CWVWebViewConfiguration*)configuration {
+  self = [super init];
+  if (self) {
+    _configuration = configuration;
+    _userScripts = [[NSMutableArray alloc] init];
+  }
+  return self;
+}
+
+- (void)addUserScript:(nonnull CWVUserScript*)userScript {
+  [_userScripts addObject:userScript];
+  [self updateEarlyPageScript];
+}
+
+- (void)removeAllUserScripts {
+  [_userScripts removeAllObjects];
+  [self updateEarlyPageScript];
+}
+
+- (nonnull NSArray<CWVUserScript*>*)userScripts {
+  return _userScripts;
+}
+
+// Updates the early page script associated with the BrowserState with the
+// content of _userScripts.
+- (void)updateEarlyPageScript {
+  NSMutableString* joinedScript = [[NSMutableString alloc] init];
+  for (CWVUserScript* script in _userScripts) {
+    [joinedScript appendString:script.source];
+    // Inserts "\n" between scripts to make it safer to join multiple scripts,
+    // in case the first script doesn't end with ";" or "\n".
+    [joinedScript appendString:@"\n"];
+  }
+  ios_web_view::WebViewEarlyPageScriptProvider::FromBrowserState(
+      _configuration.browserState)
+      .SetScript(joinedScript);
+}
+
+@end