FileReader: Make a copy of the ArrayBuffer when returning partial results.

This is to avoid accidentally ending up with multiple references to the
same underlying ArrayBuffer. The extra performance overhead of this is
minimal as usage of partial results is very rare anyway (as can be seen
on https://www.chromestatus.com/metrics/feature/timeline/popularity/2158).

Bug: 936448
Change-Id: Icd1081adc1c889829fe7fa4af9cf4440097e8854
Reviewed-on: https://chromium-review.googlesource.com/c/1492873
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636251}

third_party/blink/renderer/core/fileapi/file_reader_loader.cc

@@ -139,14 +139,16 @@ DOMArrayBuffer* FileReaderLoader::ArrayBufferResult() {
   if (!raw_data_ || error_code_ != FileErrorCode::kOK)
     return nullptr;
 
-  DOMArrayBuffer* result = DOMArrayBuffer::Create(raw_data_->ToArrayBuffer());
-  if (finished_loading_) {
-    array_buffer_result_ = result;
-    AdjustReportedMemoryUsageToV8(
-        -1 * static_cast<int64_t>(raw_data_->ByteLength()));
-    raw_data_.reset();
+  if (!finished_loading_) {
+    return DOMArrayBuffer::Create(
+        ArrayBuffer::Create(raw_data_->Data(), raw_data_->ByteLength()));
   }
-  return result;
+
+  array_buffer_result_ = DOMArrayBuffer::Create(raw_data_->ToArrayBuffer());
+  AdjustReportedMemoryUsageToV8(-1 *
+                                static_cast<int64_t>(raw_data_->ByteLength()));
+  raw_data_.reset();
+  return array_buffer_result_;
 }
 
 String FileReaderLoader::StringResult() {