Prevent unnecessary data copy in ChunkedStream.prototype.onReceiveData

This method is only invoked via `ChunkedStreamManager.prototype.sendRequest`, which currently returns data in `Uint8Array` format (since it potentially combines multiple `ArrayBuffer`s).
Hence we end up doing a short-lived, but still completely unnecessary, data copy[1] in `ChunkedStream.prototype.onReceiveData` when handling range requests. In practice this is unlikely to be a big problem by default, given that streaming is used and the (low) value of the `rangeChunkSize` API-option. (However, in custom PDF.js deployments it might affect things more.)

Given that no data copy is better than a short lived one, let's fix this small oversight and add non-production `assert`s to keep it working as intended.
This way we also improve consistency, since all other streaming and range request methods (see e.g. `BasePDFStream` and related code) only return `ArrayBuffer` data.

---
[1] Remember that `new Uint8Array(arrayBuffer)` only creates a view of the underlying `arrayBuffer`, whereas `new Uint8Array(typedArray)` actually creates a copy of the `typedArray`.

Author: Snuffleupagus

src/core/chunked_stream.js

@@ -70,6 +70,12 @@ class ChunkedStream extends Stream {
       throw new Error(`Bad end offset: ${end}`);
     }
 
+    if (typeof PDFJSDev === "undefined" || PDFJSDev.test("TESTING")) {
+      assert(
+        chunk instanceof ArrayBuffer,
+        "onReceiveData - expected an ArrayBuffer."
+      );
+    }
     this.bytes.set(new Uint8Array(chunk), begin);
     const beginChunk = Math.floor(begin / chunkSize);
     const endChunk = Math.floor((end - 1) / chunkSize) + 1;
@@ -85,6 +91,12 @@ class ChunkedStream extends Stream {
     let position = this.progressiveDataLength;
     const beginChunk = Math.floor(position / this.chunkSize);
 
+    if (typeof PDFJSDev === "undefined" || PDFJSDev.test("TESTING")) {
+      assert(
+        data instanceof ArrayBuffer,
+        "onReceiveProgressiveData - expected an ArrayBuffer."
+      );
+    }
     this.bytes.set(new Uint8Array(data), position);
     position += data.byteLength;
     this.progressiveDataLength = position;
@@ -310,7 +322,7 @@ class ChunkedStreamManager {
       if (this.aborted) {
         return; // Ignoring any data after abort.
       }
-      this.onReceiveData({ chunk: data, begin });
+      this.onReceiveData({ chunk: data.buffer, begin });
     });
   }