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This changes return type of following functions:
* MultipartUploadNewFile
* MultipartUploadExistingFile

Method calls from:
https://source.chromium.org/chromium/chromium/src/+/master:components/drive/service/drive_api_service.cc;l=159;bpv=1;bpt=1
and
https://source.chromium.org/chromium/chromium/src/+/master:components/drive/drive_uploader.cc;l=167;drc=9a7c42e7b3ce922f16b308e2b295f109b56b9fa2;bpv=1;bpt=1

Bug: 1007686
Change-Id: I363b85d59e254e1481fb4c60c29ff9ec0bdc8c00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2304727
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790931}