Reset user data directory and disk cache directory after downgrade.

components/policy/resources/policy_templates.json

 {
 # policy_templates.json - Metafile for policy templates
 #
 # The content of this file is evaluated as a Python expression.
 #
 # This file is used as input to generate the following policy templates:
 # ADM, ADMX+ADML, MCX/plist and html documentation.
 #
 # Policy templates are user interface definitions or documents about the
 # policies that can be used to configure Chrome. Each policy is a name-value
@@ skipping 1397 matching lines @@
       'example_value': '${users}/${user_name}/Chrome',
       'id': 63,
       'caption': '''Set user data directory''',
       'tags': ['local-data-access'],
       'desc': '''Configures the directory that <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> will use for storing user data.
 
       If you set this policy, <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> will use the provided directory regardless whether the user has specified the '--user-data-dir' flag or not.
 
       See https://www.chromium.org/administrators/policy-list-3/user-data-directory-variables for a list of variables that can be used.
 
-      If this policy is left not set the default profile path will be used and the user will be able to override it with the '--user-data-dir' command line flag.''',
+      If this policy is left not set the default profile path will be used and the user will be able to override it with the '--user-data-dir' command line flag.
+
+      This policy must not be set to a volume's root directory, and should not be set to a directory used for other purposes.''',

[pastarmovj, 2016/06/01 15:09:40]

Can you explain to me why is this the case? I don't think this is really
impossible only it is a bad choice I guess. Or do you mean this would cause your
profile deletion logic to wipe the hard drive? 8-|

Regardless of the discussion above I think this paragraph belongs above the
paragraph "If this policy is not set...".

[zmin, 2016/06/01 15:34:30]

No, it won't wipe the hard drive. However, there is no easy way for us to delete
a directory if it's set to root directory. Because we can't move root directory
before delete like others and delete without move causes many other issues (too
slow, partially delete).
As a result, if the |user_data_dir| or |disk_cache_dir| is set to root
directory, the removing code will simply be skipped.

Because root directory can potentially cause issues after the downgrade and it's
hard for us to explain why as we don't want to document downgrade feature
publicly. I think it's just good to tell the users to avoid using it even though
Admins can still set it at their own risk.

I'll combine this paragraph with the above one. 

[zmin, 2016/06/01 15:45:22]

Actually, I'll put it behind the paragraph of 'If you set this policy, .... " as
they are both talking about "set".

[pastarmovj, 2016/06/02 10:04:35]

I understand that we don't want to have a lengthy and potentially hard to follow
conversation here why is this the case. But at least let's have something like
"To avoid data loss this policy should not be set to a volume's root directory
or to a directory used for other purposes, because <ph
name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> manages its contents." or some
such. It still doesn't go into detail but kind of gives the idea of why is this
not a great idea. WDYT?

       'label': '''Set user data directory''',
     },
     {
       'name': 'DiskCacheDir',
       'type': 'string',
       'schema': { 'type': 'string' },
       'supported_on': ['chrome.*:13-'],
       'features': {
         'dynamic_refresh': False,
         'per_profile': False,
       },
       'example_value': '${user_home}/Chrome_cache',
       'id': 88,
       'caption': '''Set disk cache directory''',
       'tags': [],
       'desc': '''Configures the directory that <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> will use for storing cached files on the disk.
 
       If you set this policy, <ph name="PRODUCT_NAME">$1<ex>Google Chrome</ex></ph> will use the provided directory regardless whether the user has specified the '--disk-cache-dir' flag or not.
 
       See https://www.chromium.org/administrators/policy-list-3/user-data-directory-variables for a list of variables that can be used.
 
-      If this policy is left not set the default cache directory will be used and the user will be able to override it with the '--disk-cache-dir' command line flag.''',
+      If this policy is left not set the default cache directory will be used and the user will be able to override it with the '--disk-cache-dir' command line flag.
+
+      This policy must not be set to a volume's root directory, and should not be set to a directory used for other purposes.''',

[pastarmovj, 2016/06/01 15:09:40]

Ditto.

[zmin, 2016/06/01 15:34:29]

Done.

       'label': '''Set disk cache directory''',
     },
     {
       'name': 'DiskCacheSize',
       'type': 'int',
       'schema': { 'type': 'integer' },
       'supported_on': ['chrome.*:17-'],
       'features': {
         'dynamic_refresh': False,
         'per_profile': False,
@@ skipping 7190 matching lines @@
       'desc': '''Text appended in parentheses next to the policies top-level container to indicate that those policies are of the Recommended level''',
       'text': 'Default Settings (users can override)',
     },
     'doc_complex_policies_on_windows': {
       'desc': '''Text pointing the user to a help article for complex policies on Windows''',
       'text': '''encoded as a JSON string, for details see <ph name="COMPLEX_POLICIES_URL">https://www.chromium.org/administrators/complex-policies-on-windows<ex>https://www.chromium.org/administrators/complex-policies-on-windows</ex></ph>''',
     },
   },
   'placeholders': [],
 }