[Fallback Icons] Refactor FallbackIconService to be a BrowserContext-level singleton

chrome/browser/favicon/fallback_icon_service_factory.cc

+// Copyright 2015 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/favicon/fallback_icon_service_factory.h"
+
+#include "base/memory/singleton.h"
+#include "base/prefs/pref_service.h"
+#include "chrome/browser/favicon/chrome_fallback_icon_client_factory.h"
+#include "chrome/browser/profiles/profile.h"
+#include "components/favicon/core/fallback_icon_service.h"
+#include "components/keyed_service/content/browser_context_dependency_manager.h"
+
+// static
+FallbackIconService* FallbackIconServiceFactory::GetForProfile(
+    Profile* profile, ServiceAccessType sat) {
+  if (!profile->IsOffTheRecord()) {
+    return static_cast<FallbackIconService*>(
+        GetInstance()->GetServiceForBrowserContext(profile, true));
+  } else if (sat == ServiceAccessType::EXPLICIT_ACCESS) {
+    // Profile must be OffTheRecord in this case.

[beaudoin, 2015/03/26 12:53:55]

Not sure I like the phrasing, I'd go with:
// Profile is OffTheRecord with access.

[huangs, 2015/03/26 18:26:06]

The file is branched from favicon_service_factory.cc and that's what's done
there. Same with below.  Should both be fixed?

+    return static_cast<FallbackIconService*>(
+        GetInstance()->GetServiceForBrowserContext(
+            profile->GetOriginalProfile(), true));
+  }
+
+  // Profile is OffTheRecord without access.

[beaudoin, 2015/03/26 12:53:55]

Explain in the comment why this should never happen.

[huangs, 2015/03/26 18:26:06]

Will continue discussion once I uploaded with lower diff threshold.

+  NOTREACHED() << "This profile is OffTheRecord";
+  return nullptr;
+}
+
+// static
+FallbackIconServiceFactory* FallbackIconServiceFactory::GetInstance() {
+  return Singleton<FallbackIconServiceFactory>::get();
+}
+
+FallbackIconServiceFactory::FallbackIconServiceFactory()
+    : BrowserContextKeyedServiceFactory(
+        "FallbackIconService",
+        BrowserContextDependencyManager::GetInstance()) {
+  DependsOn(ChromeFallbackIconClientFactory::GetInstance());
+}
+
+FallbackIconServiceFactory::~FallbackIconServiceFactory() {}
+
+KeyedService* FallbackIconServiceFactory::BuildServiceInstanceFor(
+    content::BrowserContext* profile) const {
+  FallbackIconClient* fallback_icon_client =
+      ChromeFallbackIconClientFactory::GetForProfile(
+          static_cast<Profile*>(profile));
+  return new FallbackIconService(fallback_icon_client);
+}
+
+bool FallbackIconServiceFactory::ServiceIsNULLWhileTesting() const {
+  return true;
+}