Add 'FsNosymfollow' flag to VariationsService and chrome://flags

This flag allows for controlling whether user-supplied filesystems in
CrOS are mounted with the 'nosymfollow' option that will cause symlink
traversal to be blocked. Will be configurable through VariationsService
as well as chrome://flags.

This needs CL:1338199 to land and then a deps roll to happen before
this can land.

"sudo -u chronos dbus-send --system --type=method_call --print-reply \
--dest=org.chromium.ChromeFeaturesService \
/org/chromium/ChromeFeaturesService \
"org.chromium.ChromeFeaturesServiceInterface.IsFsNosymfollowEnabled"

Bug: 867807
Test: look at chrome://flags as well as:
Change-Id: Iffd13ebf95a2d1bb12541a8c910aa0bba18ef79f
Reviewed-on: https://chromium-review.googlesource.com/c/1338173
Reviewed-by: Dan Erat <[email protected]>
Reviewed-by: Mattias Nissler <[email protected]>
Reviewed-by: Elly Fong-Jones <[email protected]>
Commit-Queue: Micah Morton <[email protected]>
Cr-Commit-Position: refs/heads/master@{#609555}
9 files changed