build_RootFilesystemSize: change the free space threshold.

This changes the size of free space from 15 MB to 11 MB to make
reef-paladin pass.

BUG=chromium:851897
TEST=the threshold is changed from 15 MB to 11 MB.

Change-Id: Id81ac2d926282f2c61954cd8f614f8130084cf4b
Reviewed-on: https://chromium-review.googlesource.com/1096617
Reviewed-by: Lann Martin <[email protected]>
Tested-by: Aseda Aboagye <[email protected]>
diff --git a/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py b/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py
index ba29bdc..2b069d5 100644
--- a/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py
+++ b/client/site_tests/build_RootFilesystemSize/build_RootFilesystemSize.py
@@ -50,9 +50,9 @@
         if fs_type == 'squashfs':
             return
 
-        # Fail if we are running out of free space on rootfs (15 MiB or
+        # Fail if we are running out of free space on rootfs (11 MiB or
         # 2% free space).
-        required_free_space = min(15 * 1024 * 1024, used * 0.02)
+        required_free_space = min(11 * 1024 * 1024, used * 0.02)
 
         if free < required_free_space:
             raise error.TestFail('%s bytes free is less than the %s required.' %