Check the content setting type is valid.

BUG=169770
Review URL: https://codereview.chromium.org/11875013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176687 0039d316-1c4b-4281-b951-d872f2087c98
void TabSpecificContentSettings::AddBlockedResource(
    ContentSettingsType content_type,
    const std::string& resource_identifier) {
  if (!blocked_resources_[content_type].get())
    blocked_resources_[content_type].reset(new std::set<std::string>());
  blocked_resources_[content_type]->insert(resource_identifier);
}

void TabSpecificContentSettings::AddBlockedResource(
    ContentSettingsType content_type,
    const std::string& resource_identifier) {
  if (!blocked_resources_[content_type].get())
    blocked_resources_[content_type].reset(new std::set<std::string>());
  blocked_resources_[content_type]->insert(resource_identifier);
}
