use TransformStream if available

app/ece.js

@@ -1,5 +1,5 @@
 import 'buffer';
-import { transform } from './streams';
+import { transformStream } from './streams';
 
 const NONCE_LENGTH = 12;
 const TAG_LENGTH = 16;
@@ -353,9 +353,12 @@ export default class ECE {
         new BlobSlicer(this.input, this.rs, this.mode)
       );
     } else {
-      inputStream = transform(this.input, new StreamSlicer(this.rs, this.mode));
+      inputStream = transformStream(
+        this.input,
+        new StreamSlicer(this.rs, this.mode)
+      );
     }
-    return transform(
+    return transformStream(
       inputStream,
       new ECETransformer(this.mode, this.key, this.rs, this.salt)
     );

app/serviceWorker.js

@@ -1,6 +1,6 @@
 import Keychain from './keychain';
 import { downloadStream } from './api';
-import { transform } from './streams';
+import { transformStream } from './streams';
 import contentDisposition from 'content-disposition';
 
 let noSave = false;
@@ -24,7 +24,7 @@ async function decryptStream(request) {
 
     const body = await file.download.result;
 
-    const readStream = transform(body, {
+    const readStream = transformStream(body, {
       transform: (chunk, controller) => {
         file.progress += chunk.length;
         controller.enqueue(chunk);

app/streams.js

@@ -1,6 +1,9 @@
-/* global ReadableStream */
+/* global ReadableStream TransformStream */
 
-export function transform(readable, transformer) {
+export function transformStream(readable, transformer) {
+  if (typeof TransformStream === 'function') {
+    return readable.pipeThrough(new TransformStream(transformer));
+  }
   const reader = readable.getReader();
   const tstream = new ReadableStream({
     start(controller) {