fix: FileReader: Make a copy of the ArrayBuffer when returning partial results (3-0-x)

[ppontes]

Backports https://chromium-review.googlesource.com/c/chromium/src/+/1492873

• [x] PR description included and stakeholders cc'd
• [ ] Patch information is added to appropriate .patches.yaml
• [ ] script/update runs without error
• [x] PR title follows semantic [commit guidelines]

[ppontes]

Closing in favour or PR from electron/libchromiumcontent.