message: Fall back to JS-based CTR if the browser does not support CTR natively.
  Heads up that this change was originally made in cl/199854220, but it appears to have been accidentally lost in cl/200480315.
diff: @@ -52,10 +52,14 @@ const newInstance = async function(key, ivSize) {
  Validators.validateAesKeySize(key.length);
  if (Environment.IS_WEBCRYPTO_AVAILABLE) {
  + try {
  const cryptoKey = await window.crypto.subtle.importKey(
  'raw', key, {'name': 'AES-CTR', 'length': key.length}, false,
  ['encrypt', 'decrypt']);
  return new AesCtrWebCrypto(cryptoKey, ivSize);
  + } catch (error) {
  + // CTR might be unsupported in this browser. Fall back to Pure JS.
  + }
  }
  return new AesCtrPureJs(key, ivSize);
  };
  