Disable sync for Lightspeed experimentation.

We want to determine Sync's contribution to Browser startup. As such,
we will add code allowing us to disable Sync (or a part thereof) via
Finch.

The SupervisedUserService was/is not robust to disabling Sync (it
crashes). Since disabling sync is a generally supported feature
(there's a command line flag), that is now fixed.

BUG=454788

Review URL: https://codereview.chromium.org/933293005

Cr-Commit-Position: refs/heads/master@{#317642}

chrome/browser/sync/profile_sync_components_factory_impl.cc

@@ -146,6 +146,18 @@ syncer::ModelTypeSet GetDisabledTypesFromCommandLine(
   syncer::ModelTypeSet disabled_types;
   std::string disabled_types_str =
       command_line.GetSwitchValueASCII(switches::kDisableSyncTypes);
+
+  // Disable sync types experimentally to measure impact on startup time.
+  // TODO(mlerman): Remove this after the experiment. crbug.com/454788
+  std::string disable_types_finch =
+      variations::GetVariationParamValue("LightSpeed", "DisableSyncPart");
+  if (!disable_types_finch.empty()) {
+    if (disabled_types_str.empty())
+      disabled_types_str = disable_types_finch;
+    else
+      disabled_types_str += ", " + disable_types_finch;
+  }
+
   disabled_types = syncer::ModelTypeSetFromString(disabled_types_str);
   return disabled_types;
 }

chrome/browser/sync/profile_sync_service_factory.cc

@@ -32,6 +32,7 @@
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
 #include "components/signin/core/browser/profile_oauth2_token_service.h"
 #include "components/signin/core/browser/signin_manager.h"
+#include "components/variations/variations_associated_data.h"
 #include "url/gurl.h"
 
 #if defined(ENABLE_EXTENSIONS)
@@ -50,6 +51,15 @@ ProfileSyncService* ProfileSyncServiceFactory::GetForProfile(
   if (!ProfileSyncService::IsSyncEnabled())
     return NULL;
 
+  // Disable sync experimentally to measure impact on startup time. Supervised
+  // users are unaffected, since supervised users rely completely on sync.
+  // TODO(mlerman): Remove this after the experiment. crbug.com/454788
+  if (!profile->IsSupervised() &&
+      !variations::GetVariationParamValue("LightSpeed", "DisableSync")
+           .empty()) {
+    return NULL;
+  }
+
   return static_cast<ProfileSyncService*>(
       GetInstance()->GetServiceForBrowserContext(profile, true));
 }