[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 AppListSyncableService::SyncStarted() {
  if (sync_processor_.get())
    return true;
  if (flare_.is_null()) {
    VLOG(1) << this << ": SyncStarted: Flare.";
    flare_ = sync_start_util::GetFlareForSyncableService(profile_->GetPath());
    flare_.Run(syncer::APP_LIST);
  }
  return false;
}

bool AppListSyncableService::SyncStarted() {
  if (sync_processor_.get())
    return true;
  if (flare_.is_null()) {
    VLOG(1) << this << ": SyncStarted: Flare.";
    flare_ = sync_start_util::GetFlareForSyncableService(profile_->GetPath());
    flare_.Run(syncer::APP_LIST);
  }
  return false;
}
