build_RootFilesystemSize: shrink to 20MiB required

Lumpy paladin failed repeatedly. Lower this for the time being until
the space issue is resolved.

BUG=chromium:317903
TEST=None

Change-Id: I940ad6ab4b6a385f08ffa52d73f48718b9601bb3
Previous-Reviewed-on: https://chromium-review.googlesource.com/178541
(cherry picked from commit 481cb8ff22e2e70323cd64d624ae0587719b672e)
Reviewed-on: https://chromium-review.googlesource.com/180228
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 a3a5289..40330fa 100644
--- a/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py
+++ b/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py
@@ -31,7 +31,7 @@
         self.write_perf_keyval({'bytes_rootfs_test': float(used)})
 
         # Fail if we are running out of free space on rootfs.
-        required_free_space = 40 * 1024 * 1024
+        required_free_space = 20 * 1024 * 1024
 
         if int(free) < required_free_space:
             raise error.TestFail('%s bytes free is less than the %s required.' %