[Native File System] Refactor write methods from NFSFileHandle to NFSFileWriter

This CL moves write-related operations to NFSFileWriter. This makes
the backend and interface closer to the current shape of the spec.

Bug: 968550
Change-Id: Ia2ccd97416b478eddf210118132519eef2b16040
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672026
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Commit-Queue: Olivier Yiptong <oyiptong@chromium.org>
Auto-Submit: Olivier Yiptong <oyiptong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672105}
14 files changed