Ensure ContentSettingsStore::RegisterExtension is called for components

[bbondy]

Fix https://github.com/brave/brave/issues/81

Note for reviewing that the added call to extension_prefs_->OnExtensionInstalled is always called right away in AddNewOrUpdatedExtension but for AddExtension it is never called:

void ExtensionService::AddComponentExtension(const Extension* extension) {
  const std::string old_version_string(
      extension_prefs_->GetVersionString(extension->id()));
  const base::Version old_version(old_version_string);

  VLOG(1) << "AddComponentExtension " << extension->name();
  if (!old_version.IsValid() || old_version != *extension->version()) {
    VLOG(1) << "Component extension " << extension->name() << " ("
        << extension->id() << ") installing/upgrading from '"
        << old_version_string << "' to " << extension->version()->GetString();

    // TODO(crbug.com/696822): If needed, add support for Declarative Net
    // Request to component extensions and pass the ruleset checksum here.
    AddNewOrUpdatedExtension(
        extension, Extension::ENABLED, extensions::kInstallFlagNone,
        syncer::StringOrdinal(), std::string(), base::nullopt);
    return;
  }

  AddExtension(extension);

This seem to only be needed for AddComponentExtension and not extensions because the extensions are always loaded before the default components are added.

Let me know if you can think of a better way to fix this. I had trouble finding a cleaner way.

[darkdh]

Lgtm