Don't request the default pref store from the pref service.

The browser already has all the defaults and ignores the returned pref
store connection so this is just wasting time at startup.

Bug: 654988
Change-Id: I05a6af935fd44f5ea9d37cef71e3acf3d782e4d4
Reviewed-on: https://chromium-review.googlesource.com/494407Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#468935}

chrome/browser/prefs/browser_prefs.cc

@@ -814,7 +814,12 @@ std::set<PrefValueStore::PrefStoreType> ExpectedPrefStores() {
 
 std::set<PrefValueStore::PrefStoreType> InProcessPrefStores() {
   auto pref_stores = ExpectedPrefStores();
-  pref_stores.erase(PrefValueStore::DEFAULT_STORE);
+  // Until we have a distinction between owned and unowned prefs, we always have
+  // default values for all prefs locally. Since we already have the defaults it
+  // would be wasteful to request them from the service by connecting to the
+  // DEFAULT_STORE.
+  // TODO(sammc): Once we have this distinction, connect to the default pref
+  // store here (by erasing it from |pref_stores|).
   pref_stores.erase(PrefValueStore::USER_STORE);
   return pref_stores;
 }

services/preferences/pref_store_manager_impl.cc

@@ -76,6 +76,13 @@ void ConnectionBarrier::Create(
 
 void ConnectionBarrier::Init(const PrefStorePtrs& pref_store_ptrs,
                              const std::vector<std::string>& observed_prefs) {
+  if (pref_store_ptrs.empty()) {
+    // After this method exits |this| will get destroyed so it's safe to move
+    // out the members.
+    callback_.Run(std::move(persistent_pref_store_connection_),
+                  std::move(connections_));
+    return;
+  }
   for (const auto& ptr : pref_store_ptrs) {
     ptr.second->AddObserver(
         observed_prefs,