Auto merge of #12258 - izgzhen:remove-data-slice, r=Manishearth

Remove DataSlice, fix #12249

r? @Manishearth

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #12249

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- [ ] These changes do not require tests because _____

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components/script/dom/htmlformelement.rs

@@ -324,9 +324,9 @@ impl HTMLFormElement {
                         content_disposition,
                         content_type));
 
-                    let slice = f.upcast::<Blob>().get_slice_or_empty();
+                    let bytes = &f.upcast::<Blob>().get_bytes().unwrap_or(vec![])[..];
 
-                    let decoded = encoding.decode(&slice.get_bytes(), DecoderTrap::Replace)
+                    let decoded = encoding.decode(bytes, DecoderTrap::Replace)
                                           .expect("Invalid encoding in file");
                     result.push_str(&decoded);
                 }