GitLab

Implements method to create a FileWriter for a FileSystemFileHandle, as
well as fixes a bug in the FileSystemWriter implementation.

Bug: 872460, 872465
Change-Id: Idfd7d2b486a7f2e1b0ec3f996ba940e10236b764
Reviewed-on: https://chromium-review.googlesource.com/1185624Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586335}