Remove NetExportFileWriter's dependence on the FILE thread.

Bug: 689520
Change-Id: If82f056dfe2dbf657d2831fefc3446194c83ef17
Reviewed-on: https://chromium-review.googlesource.com/588134
Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Eric Roman <eroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490108}
5 files changed