Remove most occurences of PrefRegistrySyncable::UNSYNCABLE_PREF

Now that we have added PrefRegistry registration functions that do not need to
accept a flags argument, passing PrefRegistrySyncable::UNSYNCABLE_PREF is
redundant and we can remove it altogether.

Ran the following command:
for x in $(git ls-files | grep -E "\.(cc|h|mm)$") ; do perl -0777 -i -pe 's/,[ \n]*(user_prefs::)+PrefRegistrySyncable::UNSYNCABLE_PREF//igs' $x ; done && git cl format

There are still some occurences that need to be manually fixed.

BUG=476800

Review URL: https://codereview.chromium.org/1102733002

Cr-Commit-Position: refs/heads/master@{#326993}
diff --git a/chrome/browser/pepper_flash_settings_manager.cc b/chrome/browser/pepper_flash_settings_manager.cc
index 7dad619..eb64b49f 100644
--- a/chrome/browser/pepper_flash_settings_manager.cc
+++ b/chrome/browser/pepper_flash_settings_manager.cc
@@ -970,15 +970,9 @@
 // static
 void PepperFlashSettingsManager::RegisterProfilePrefs(
     user_prefs::PrefRegistrySyncable* registry) {
-  registry->RegisterBooleanPref(
-      prefs::kDeauthorizeContentLicenses,
-      false,
-      user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
+  registry->RegisterBooleanPref(prefs::kDeauthorizeContentLicenses, false);
 
-  registry->RegisterBooleanPref(
-      prefs::kPepperFlashSettingsEnabled,
-      true,
-      user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
+  registry->RegisterBooleanPref(prefs::kPepperFlashSettingsEnabled, true);
 }
 
 uint32 PepperFlashSettingsManager::DeauthorizeContentLicenses(