Fix BOM handling in multipart form data fetching

Request and Response's formData() multipart/form-data parsing uses
"UTF-8 decode without BOM" which should not strip BOMs.

Spec issue: https://github.com/whatwg/fetch/issues/650
Spec fix: https://github.com/whatwg/fetch/pull/915
WPT fix: https://github.com/web-platform-tests/wpt/pull/17589

Bug: 796192
Change-Id: I24721899006a8c09cf657b3638f1cb17e7953171
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796367
Commit-Queue: Eero Häkkinen <eero.hakkinen@intel.com>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712496}

third_party/blink/renderer/core/fetch/fetch_data_loader.cc

@@ -399,7 +399,7 @@ class FetchDataLoaderAsFormData final : public FetchDataLoader,
       } else {
         if (!string_decoder_) {
           string_decoder_ = std::make_unique<TextResourceDecoder>(
-              TextResourceDecoderOptions::CreateUTF8Decode());
+              TextResourceDecoderOptions::CreateUTF8DecodeWithoutBOM());
         }
         string_builder_.reset(new StringBuilder);
       }