Enable file_system_provider::Service in incognito.

There is no need to limit the FSP API to non-incognito only. Especially, such
limitation was causing crashes in Files app running in incognito, which
assumed the service exists, but it didn't.

With this CL the service is created for both non-incognito and incognito pages.
Both instances are *same*, so mounted providers will be shared between incognito
and non-incognito windows.

TEST=Tested manually by launching with --incognito
BUG=487901

Review URL: https://codereview.chromium.org/1138423002

Cr-Commit-Position: refs/heads/master@{#329792}

chrome/browser/chromeos/file_system_provider/service_factory.cc

@@ -6,6 +6,7 @@
 
 #include "chrome/browser/chromeos/file_system_provider/service.h"
 #include "chrome/browser/extensions/extension_system_factory.h"
+#include "chrome/browser/profiles/incognito_helpers.h"
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
 #include "extensions/browser/extension_registry.h"
 #include "extensions/browser/extension_registry_factory.h"
@@ -48,5 +49,10 @@ KeyedService* ServiceFactory::BuildServiceInstanceFor(
 
 bool ServiceFactory::ServiceIsCreatedWithBrowserContext() const { return true; }
 
+content::BrowserContext* ServiceFactory::GetBrowserContextToUse(
+    content::BrowserContext* context) const {
+  return chrome::GetBrowserContextRedirectedInIncognito(context);
+}
+
 }  // namespace file_system_provider
 }  // namespace chromeos

chrome/browser/chromeos/file_system_provider/service_factory.h

@@ -41,6 +41,8 @@ class ServiceFactory : public BrowserContextKeyedServiceFactory {
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* profile) const override;
   bool ServiceIsCreatedWithBrowserContext() const override;
+  content::BrowserContext* GetBrowserContextToUse(
+      content::BrowserContext* context) const override;
 
   DISALLOW_COPY_AND_ASSIGN(ServiceFactory);
 };