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>
diff --git a/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py b/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py
index 0626ee9..a3a5289 100644
--- a/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py
+++ b/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py
@@ -10,18 +10,19 @@
     def run_once(self):
         """Run the free space on rootfs test."""
 
-        # Report the production size
+        # Report the production size.
         f = open('/root/bytes-rootfs-prod', 'r')
         self.write_perf_keyval({'bytes_rootfs_prod': float(f.read())})
         f.close()
 
-        # Report the current (test) size
+        # Report the current (test) size.
         (status, output) = commands.getstatusoutput(
             'df -B1 / | tail -1')
         if status != 0:
             raise error.TestFail('Could not get size of rootfs')
 
         # Expected output format:
+        #                Total      Used Available
         # rootfs    1056858112 768479232 288378880 73% /
         output_columns = output.split()
         used = output_columns[2]
@@ -29,9 +30,9 @@
 
         self.write_perf_keyval({'bytes_rootfs_test': float(used)})
 
-        # Warn if we are running out of free space on rootfs.
-        required_free_space = 50 * 1024 * 1024
+        # Fail if we are running out of free space on rootfs.
+        required_free_space = 40 * 1024 * 1024
 
         if int(free) < required_free_space:
-          raise error.TestFail('%s bytes free is less than the %s required.' %
-                               (free, required_free_space))
+            raise error.TestFail('%s bytes free is less than the %s required.' %
+                                 (free, required_free_space))