cros_disks: Handle the case when chmod() is not implemented by filesystems.

Some filesystem implementations do not support chmod(). This CL modifies
FilesystemTestFile in cros_disks.py to ignore chmod() errors raised as
OSError.

BUG=chromium-os:39041
TEST=Run platform_CrosDisksFilesystem with exFAT tests.

Change-Id: I5ca4c7f22ad4227f4ce2ba7c7153a6ea1757580d
Reviewed-on: https://gerrit.chromium.org/gerrit/43563
Reviewed-by: Chris Sosa <[email protected]>
Commit-Queue: Ben Chan <[email protected]>
Tested-by: Ben Chan <[email protected]>
1 file changed