Auto merge of #13729 - cynicaldevil:readAsArrayBuffer, r=Ms2ger

Implemented FileReader::readAsArrayBuffer

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #12555

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- [X] There are tests for these changes

There's still some small issues, but I suppose most of the work is done:
- test-tidy mentions a `method declared in webidl is missing a comment with a specification link` for the `getResult` method.
- I get an 'unused code' warning for code present in `UnionTypes.rs`, which is auto-generated.

Passing tests:
- [x]  `FileAPI/reading-data-section/filereader_result.html`
- [x]  `FileAPI/reading-data-section/filereader_readAsArrayBuffer.html`
- [x]  `FileAPI/idlharness.html`
- [ ]  `FileAPI/reading-data-section/FileReader-multiple-reads.html`

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components/script/dom/webidls/FileReader.webidl

@@ -4,13 +4,13 @@
 
  // http://dev.w3.org/2006/webapi/FileAPI/#APIASynch
 
-//typedef (DOMString or ArrayBuffer) FileReaderResult;
+typedef (DOMString or object) FileReaderResult;
 [Constructor, Exposed=(Window,Worker)]
 interface FileReader: EventTarget {
 
   // async read methods
-  //[Throws]
-  //void readAsArrayBuffer(Blob blob);
+  [Throws]
+  void readAsArrayBuffer(Blob blob);
   [Throws]
   void readAsText(Blob blob, optional DOMString label);
   [Throws]
@@ -25,8 +25,7 @@ interface FileReader: EventTarget {
   readonly attribute unsigned short readyState;
 
   // File or Blob data
-  //readonly attribute FileReaderResult? result;
-  readonly attribute DOMString? result;
+  readonly attribute FileReaderResult? result;
 
   readonly attribute DOMException? error;