Revert "deploy_chrome: convert umount logic away from shell=True"

This reverts commit e7dc3d57cbebdd9866e9d488990c70f7b13ccd59.

It has broken the devserver provisioning.

1/2 

BUG=chromium:776099

Change-Id: I7fc910c63d774779a0116b78a924bbc0aa3756e3
Reviewed-on: https://chromium-review.googlesource.com/734901
Reviewed-by: Richard Barnette <jrbarnette@google.com>
Commit-Queue: David Haddock <dhaddock@chromium.org>
Tested-by: David Haddock <dhaddock@chromium.org>
diff --git a/scripts/deploy_chrome.py b/scripts/deploy_chrome.py
index ef112c7..9219879 100644
--- a/scripts/deploy_chrome.py
+++ b/scripts/deploy_chrome.py
@@ -318,10 +318,8 @@
     self.device.RunCommand(['mkdir', '-p', '--mode', '0775',
                             self.options.mount_dir])
     # Umount the existing mount on mount_dir if present first
-    ret = self.device.RunCommand(['mountpoint', '-q', self.options.mount_dir],
-                                 error_code_ok=True)
-    if ret.returncode == 0:
-      self.device.RunCommand(['umount', self.options.mount_dir])
+    cmd = 'if mountpoint -q %(dir)s; then umount %(dir)s; fi'
+    self.device.RunCommand(cmd % {'dir': self.options.mount_dir}, shell=True)
     self.device.RunCommand(['mount', '--rbind', self.options.target_dir,
                             self.options.mount_dir])
     # Chrome needs partition to have exec and suid flags set