[on-hold][subresource_filter] Add ActivationTracker to track all activated contents

chrome/browser/subresource_filter/subresource_filter_activation_tracker.cc

+// 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.
+
+#include "chrome/browser/subresource_filter/subresource_filter_activation_tracker.h"
+
+#include <algorithm>
+
+#include "base/logging.h"
+
+SubresourceFilterActivationTracker::SubresourceFilterActivationTracker() {}
+SubresourceFilterActivationTracker::~SubresourceFilterActivationTracker() {}
+
+void SubresourceFilterActivationTracker::OnWebContentsActivated(
+    content::WebContents* web_contents,
+    bool activated) {
+  DCHECK(web_contents);
+  if (activated) {
+    activated_contents_.insert(web_contents);
+  } else {
+    activated_contents_.erase(web_contents);
+  }
+}
+
+void SubresourceFilterActivationTracker::OnWebContentsDestroyed(
+    content::WebContents* web_contents) {
+  OnWebContentsActivated(web_contents, false /* activated */);
+}
+
+bool SubresourceFilterActivationTracker::HasActivatedWebContents() const {
+  return !activated_contents_.empty();
+}