[Fuchsia] Add extra required File open flags in TouchFile().

Modifying file or directory attributes requires the Fuchsia channel
backing the file-descriptor to have write-permission.  Update the
TouchFile() utility to set the necessary File:FLAG_* value to get that
access on the target file, or directory, as appropriate.

TBR=sergeyu

Bug: 947802
Change-Id: Ib00b04b5ef8fc85d730f20ac4233422b07052f2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1547129
Commit-Queue: Wez <wez@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646120}

• base/files/file_util.cc[diff]