[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}
bool AppResult::RunExtensionEnableFlow() {
  if (extensions::util::IsAppLaunchableWithoutEnabling(app_id_, profile_))
    return false;

  if (!extension_enable_flow_) {
    controller_->OnShowChildDialog();

    extension_enable_flow_.reset(new ExtensionEnableFlow(
        profile_, app_id_, this));
    extension_enable_flow_->StartForNativeWindow(
        controller_->GetAppListWindow());
  }
  return true;
}

bool AppResult::RunExtensionEnableFlow() {
  if (extensions::util::IsAppLaunchableWithoutEnabling(app_id_, profile_))
    return false;

  if (!extension_enable_flow_) {
    controller_->OnShowChildDialog();

    extension_enable_flow_.reset(new ExtensionEnableFlow(
        profile_, app_id_, this));
    extension_enable_flow_->StartForNativeWindow(
        controller_->GetAppListWindow());
  }
  return true;
}
