build_RootFilesystemSize: fix comments and shrink to 40MiB required

This makes thing match reality.

Also shrink the required freespace to 40MiB so the tree will re-open.

BUG=chromium:317903
TEST=None

Change-Id: Idcfc7f092dbdb983f3deec4d129a1b796ee4474a
Previous-Reviewed-on: https://chromium-review.googlesource.com/176381
(cherry picked from commit fdc69929d7820e89722fe074146410bb95c8e061)
Reviewed-on: https://chromium-review.googlesource.com/180227
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
Tested-by: Yu-Ju Hong <yjhong@chromium.org>