Add visualization for Chrome configuration changes (fixes #3892)

- Add new API to retrieve/observe configuration values.
- cefclient: Add https://tests/config to inspect configuration
  values in real time.

libcef/browser/prefs/pref_helper.cc

@@ -6,6 +6,7 @@
 
 #include "base/notreached.h"
 #include "base/strings/stringprintf.h"
+#include "cef/include/cef_preference.h"
 #include "cef/libcef/browser/thread_util.h"
 #include "cef/libcef/common/values_impl.h"
 #include "components/prefs/pref_service.h"
@@ -118,4 +119,145 @@ bool SetPreference(PrefService* pref_service,
   return true;
 }
 
+class RegistrationImpl final : public Registration, public CefRegistration {
+ public:
+  RegistrationImpl(Registrar* registrar,
+                   const CefString& name,
+                   CefRefPtr<CefPreferenceObserver> observer)
+      : registrar_(registrar), name_(name), observer_(observer) {
+    DCHECK(registrar_);
+    DCHECK(observer_);
+  }
+
+  RegistrationImpl(const RegistrationImpl&) = delete;
+  RegistrationImpl& operator=(const RegistrationImpl&) = delete;
+
+  ~RegistrationImpl() override {
+    CEF_REQUIRE_UIT();
+    if (registrar_) {
+      registrar_->RemoveObserver(name_.ToString(), this);
+    }
+  }
+
+  void Detach() override {
+    registrar_ = nullptr;
+    observer_ = nullptr;
+  }
+
+  void RunCallback() const override { RunCallback(name_); }
+
+  void RunCallback(const CefString& name) const override {
+    observer_->OnPreferenceChanged(name);
+  }
+
+ private:
+  raw_ptr<Registrar> registrar_;
+  CefString name_;
+  CefRefPtr<CefPreferenceObserver> observer_;
+
+  IMPLEMENT_REFCOUNTING_DELETE_ON_UIT(RegistrationImpl);
+};
+
+Registrar::~Registrar() {
+  RemoveAll();
+}
+
+void Registrar::Init(PrefService* service) {
+  DCHECK(service);
+  DCHECK(IsEmpty() || service_ == service);
+  service_ = service;
+}
+
+void Registrar::Reset() {
+  RemoveAll();
+  service_ = nullptr;
+}
+
+void Registrar::RemoveAll() {
+  if (!name_observers_.empty()) {
+    for (auto& [name, registrations] : name_observers_) {
+      service_->RemovePrefObserver(name, this);
+      for (auto& registration : registrations) {
+        registration.Detach();
+      }
+    }
+    name_observers_.clear();
+  }
+
+  if (!all_observers_.empty()) {
+    service_->RemovePrefObserverAllPrefs(this);
+    for (auto& registration : all_observers_) {
+      registration.Detach();
+    }
+    all_observers_.Clear();
+  }
+}
+
+bool Registrar::IsEmpty() const {
+  return name_observers_.empty() && all_observers_.empty();
+}
+
+CefRefPtr<CefRegistration> Registrar::AddObserver(
+    const CefString& name,
+    CefRefPtr<CefPreferenceObserver> observer) {
+  CHECK(service_);
+
+  RegistrationImpl* impl = new RegistrationImpl(this, name, observer);
+
+  if (name.empty()) {
+    if (all_observers_.empty()) {
+      service_->AddPrefObserverAllPrefs(this);
+    }
+    all_observers_.AddObserver(impl);
+  } else {
+    const std::string& name_str = name.ToString();
+    if (!name_observers_.contains(name_str)) {
+      service_->AddPrefObserver(name_str, this);
+    }
+    name_observers_[name_str].AddObserver(impl);
+  }
+
+  return impl;
+}
+
+void Registrar::RemoveObserver(std::string_view name,
+                               Registration* registration) {
+  CHECK(service_);
+
+  if (name.empty()) {
+    all_observers_.RemoveObserver(registration);
+    if (all_observers_.empty()) {
+      service_->RemovePrefObserverAllPrefs(this);
+    }
+  } else {
+    auto it = name_observers_.find(std::string(name));
+    DCHECK(it != name_observers_.end());
+    it->second.RemoveObserver(registration);
+    if (it->second.empty()) {
+      name_observers_.erase(it);
+      service_->RemovePrefObserver(name, this);
+    }
+  }
+}
+
+void Registrar::OnPreferenceChanged(PrefService* service,
+                                    std::string_view pref_name) {
+  std::string pref_name_str(pref_name);
+  if (!name_observers_.empty()) {
+    auto it = name_observers_.find(pref_name_str);
+    if (it != name_observers_.end()) {
+      for (Registration& registration : it->second) {
+        registration.RunCallback();
+      }
+    }
+  }
+
+  if (!all_observers_.empty()) {
+    CefString name_str(pref_name_str);
+    for (Registration& registration : all_observers_) {
+      registration.RunCallback(name_str);
+    }
+  }
+}
+
 }  // namespace pref_helper