(#5140) - remove use of buffer.toArrayBuffer()

buffer dropped support for toArrayBuffer
(see https://github.com/feross/buffer/issues/90). However, all the
browsers we support have typed array support so the storedObject will
be implemented as a typed array from which we can directly construct
a Blob.

src/adapters/leveldb/readAsBlobOrBuffer-browser.js

diff --git a/src/adapters/leveldb/readAsBlobOrBuffer-browser.js b/src/adapters/leveldb/readAsBlobOrBuffer-browser.js
index 9906a01..d33c202 100644
--- a/src/adapters/leveldb/readAsBlobOrBuffer-browser.js
+++ b/src/adapters/leveldb/readAsBlobOrBuffer-browser.js
@@ -2,8 +2,10 @@
 
 function readAsBlobOrBuffer(storedObject, type) {
   // In the browser, we've stored a binary string. This now comes back as a
-  // browserified Node-style Buffer, but we want a Blob instead.
-  return createBlob([storedObject.toArrayBuffer()], {type: type});
+  // browserified Node-style Buffer (implemented as a typed array),
+  // but we want a Blob instead.
+  var byteArray = new Uint8Array(storedObject);
+  return createBlob([byteArray], {type: type});
 }
 
 export default readAsBlobOrBuffer;
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