[Sync] Adds a new switch for enabling the new local server backend.

components/browser_sync/profile_sync_service.cc

Index: components/browser_sync/profile_sync_service.cc
diff --git a/components/browser_sync/profile_sync_service.cc b/components/browser_sync/profile_sync_service.cc
index f013dbc8d840baee3f730dd4501d34a646be487b..ff9a28f5ea1c620e35986eee68c61c16f42e5616 100644
--- a/components/browser_sync/profile_sync_service.cc
+++ b/components/browser_sync/profile_sync_service.cc
@@ -20,6 +20,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/metrics/histogram.h"
+#include "base/path_service.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/stringprintf.h"
@@ -118,6 +119,9 @@ typedef GoogleServiceAuthError AuthError;
 
 const char kSyncUnrecoverableErrorHistogram[] = "Sync.UnrecoverableErrors";
 
+const base::FilePath::StringType kLoopbackServerBackendFilename =
+    FILE_PATH_LITERAL("profile.pb");
+
 const net::BackoffEntry::Policy kRequestAccessTokenBackoffPolicy = {
     // Number of initial errors (in sequence) to ignore before applying
     // exponential back-off rules.
@@ -502,6 +506,36 @@ void ProfileSyncService::InitializeBackend(bool delete_stale_data) {
   if (delete_stale_data)
     ClearStaleErrors();
 
+  bool enable_local_sync_backend = false;
+  base::FilePath local_sync_backend_folder;
+#if defined(OS_WIN)
+  enable_local_sync_backend = base::CommandLine::ForCurrentProcess()->HasSwitch(
+      switches::kEnableLocalSyncBackend);
+  if (base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kLocalSyncBackendDir)) {
+    local_sync_backend_folder =
+        base::CommandLine::ForCurrentProcess()->GetSwitchValuePath(
+            switches::kLocalSyncBackendDir);
+  } else {
+    // TODO(pastarmovj): Add DIR_ROAMING_USER_DATA to PathService to simplify
+    // this code and move the logic in its right place. See crbug/657810.
+    CHECK(
+        base::PathService::Get(base::DIR_APP_DATA, &local_sync_backend_folder));
+    local_sync_backend_folder =
+        local_sync_backend_folder.Append(FILE_PATH_LITERAL("Chrome/User Data"));
+  }
+  // This code as it is now will assume the same profile order is present on all
+  // machines, which is not a given. It is to be defined if only the Default
+  // profile should get this treatment or all profile as is the case now. The
+  // solution for now will be to assume profiles are created in the same order
+  // on all machines and in the future decide if only the Default one should be
+  // considered roamed.
+  local_sync_backend_folder =
+      local_sync_backend_folder.Append(base_directory_.BaseName());
+  local_sync_backend_folder =
+      local_sync_backend_folder.Append(kLoopbackServerBackendFilename);
+#endif  // defined(OS_WIN)
+
   SyncBackendHost::HttpPostProviderFactoryGetter
       http_post_provider_factory_getter =
           base::Bind(&syncer::NetworkResources::GetHttpPostProviderFactory,
@@ -511,9 +545,9 @@ void ProfileSyncService::InitializeBackend(bool delete_stale_data) {
   backend_->Initialize(
       this, std::move(sync_thread_), db_thread_, file_thread_,
       GetJsEventHandler(), sync_service_url_, local_device_->GetSyncUserAgent(),
-      credentials, delete_stale_data,
-      std::unique_ptr<syncer::SyncManagerFactory>(
-          new syncer::SyncManagerFactory()),
+      credentials, delete_stale_data, enable_local_sync_backend,
+      local_sync_backend_folder, std::unique_ptr<syncer::SyncManagerFactory>(
+                                     new syncer::SyncManagerFactory()),
       MakeWeakHandle(sync_enabled_weak_factory_.GetWeakPtr()),
       base::Bind(syncer::ReportUnrecoverableError, channel_),
       http_post_provider_factory_getter, std::move(saved_nigori_state_));