Code:  void UninstallExtension(ExtensionService* service, const std::string& id) {
  if (service && service->GetInstalledExtension(id)) {
    service->UninstallExtension(id,
                                extensions::UNINSTALL_REASON_SYNC,
                                base::Bind(&base::DoNothing),
                                NULL);
   }
 }

Commit Message: [Extensions] Add GetInstalledExtension() method to ExtensionRegistry

This CL adds GetInstalledExtension() method to ExtensionRegistry and
uses it instead of deprecated ExtensionService::GetInstalledExtension()
in chrome/browser/ui/app_list/.

Part of removing the deprecated GetInstalledExtension() call
from the ExtensionService.

BUG=489687

Review URL: https://codereview.chromium.org/1130353010

Cr-Commit-Position: refs/heads/master@{#333036}
CWE ID: 
 void UninstallExtension(ExtensionService* service, const std::string& id) {
  if (service) {
    ExtensionService::UninstallExtensionHelper(
        service, id, extensions::UNINSTALL_REASON_SYNC);
   }
 }
