The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c334558eef67b1216278c0f505a49a0948bc707d

commit c334558eef67b1216278c0f505a49a0948bc707d
Author: yamaguchi <yamaguchi@chromium.org>
Date: Mon Nov 28 13:12:45 2016

Revise the description of ExternalStorageReadOnly to make it clearer.
This is a follow-up of https://codereview.chromium.org/2526463002/ .

Here is the actual behavior of the system:

X=ExternaStorageDisabled
Y=ExternalStorageReadOnly

  X     Y
False  False : Users can read from and write to external storage media.
False  True : Users can read files in external storage, but cannot write to there.
True  False : Users cannot access external storage at all.
True  True : Users cannot access external storage at all.

If any of the policy value is not configured (=not exist), it is equivalent to false.

If ExternaStorageDisabled = true, any access to removable storage media are denied. In this case, ExternalStorageReadOnly will not be considered because the storage media devices itself will never be accessible by the user.

BUG= 669004 

Review-Url: https://codereview.chromium.org/2536593002
Cr-Commit-Position: refs/heads/master@{#434644}

[modify] https://crrev.com/c334558eef67b1216278c0f505a49a0948bc707d/components/policy/resources/policy_templates.json