[iOS] Upstream DomAlteringLock.

ios/chrome/browser/web/dom_altering_lock.mm

+// Copyright 2014 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/chrome/browser/web/dom_altering_lock.h"
+
+DEFINE_WEB_STATE_USER_DATA_KEY(DOMAlteringLock);
+
+DOMAlteringLock::DOMAlteringLock(web::WebState* web_state) {
+}
+
+DOMAlteringLock::~DOMAlteringLock() {
+}
+
+void DOMAlteringLock::Acquire(id<DOMAltering> feature,
+                              ProceduralBlockWithBool lockAction) {
+  DCHECK([NSThread isMainThread]);

[sdefresne, 2015/02/17 13:20:55]

Can you use web::WebThread? And everywhere else.

+  if (current_dom_altering_feature_.get() == feature) {
+    lockAction(YES);
+    return;
+  }
+  if (current_dom_altering_feature_) {
+    if (![current_dom_altering_feature_ canReleaseDOMLock]) {
+      lockAction(NO);
+      return;
+    }
+    [current_dom_altering_feature_ releaseDOMLockWithCompletionHandler:^() {
+      DCHECK([NSThread isMainThread]);
+      DCHECK(current_dom_altering_feature_.get() == nil)
+          << "The lock must be released before calling the completion handler.";
+      current_dom_altering_feature_.reset(feature);
+      lockAction(YES);
+    }];
+    return;
+  }
+  current_dom_altering_feature_.reset(feature);
+  lockAction(YES);
+}
+
+// Release the lock on the DOM tree.
+void DOMAlteringLock::Release(id<DOMAltering> feature) {
+  DCHECK([NSThread isMainThread]);
+  if (current_dom_altering_feature_.get() == feature)
+    current_dom_altering_feature_.reset();
+}