Reland "Include compatibility checks when submitting policy_templates.json."

The original revert was done prematurely to fix a build break but this change was not the cause of the build break.

This reverts commit 6c8601ff.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Revert "Include compatibility checks when submitting policy_templates.json."
> 
> This reverts commit d44d94c6.
> 
> Reason for revert: I think this is the cause for this tree-closing failure:
> https://ci.chromium.org/p/chromium/builders/ci/win32-archive-rel/7768
> 
> Original change's description:
> > Include compatibility checks when submitting policy_templates.json.
> > 
> > This will change the presubmit checks for policy_templates.json to
> > ensure that policy changes are backwards compatible with policies
> > definitions that have already shipped.
> > 
> > Bug: 1026330
> > Change-Id: If054d2376397432c66d948eaf0d2d42d5db13a0b
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925190
> > Reviewed-by: Sergey Poromov <poromov@chromium.org>
> > Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
> > Commit-Queue: Tien Mai <tienmai@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#721123}
> 
> TBR=pastarmovj@chromium.org,poromov@chromium.org,tienmai@chromium.org
> 
> Change-Id: I0ac66ee107a0ad25675e149192b47868653c525c
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 1026330
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949265
> Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
> Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#721139}

TBR=pastarmovj@chromium.org,poromov@chromium.org,takumif@chromium.org,tienmai@chromium.org

Change-Id: I4ad04786c13ad4cf8be57399489e0bf311cd6fb6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1026330
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1950744Reviewed-by: Tien Mai <tienmai@chromium.org>
Commit-Queue: Tien Mai <tienmai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721471}

components/policy/resources/PRESUBMIT.py

@@ -35,8 +35,42 @@ def _CheckPolicyTemplatesSyntax(input_api, output_api):
       device_policy_proto_path = input_api.os_path.join(
           local_path, '../proto/chrome_device_policy.proto')
       args = ["--device_policy_proto_path=" + device_policy_proto_path]
+
+      root = input_api.change.RepositoryRoot()
+
+      current_version = None
+      original_file_contents = None
+
+      # Check if there is a tag that allows us to bypass compatibility checks.
+      # This can be used in situations where there is a bug in the validation
+      # code or if a policy change needs to urgently be submitted.
+      if not input_api.change.tags.get('BYPASS_POLICY_COMPATIBILITY_CHECK'):
+        # Get the current version from the VERSION file so that we can check
+        # which policies are un-released and thus can be changed at will.
+        try:
+          version_path = input_api.os_path.join(
+          root, 'chrome', 'VERSION')
+          with open(version_path, "rb") as f:
+            current_version = int(f.readline().split("=")[1])
+            print ('Checking policies against current version: ' +
+              current_version)
+        except:
+          pass
+
+        # Get the original file contents of the policy file so that we can check
+        # the compatibility of template changes in it
+        template_path = input_api.os_path.join(
+          root, 'components', 'policy', 'resources', 'policy_templates.json')
+        affected_files = input_api.change.AffectedFiles()
+        template_affected_file = next(iter(f \
+          for f in affected_files if f.AbsoluteLocalPath() == template_path))
+        if template_affected_file is not None:
+          original_file_contents = \
+            '\n'.join(template_affected_file.OldContents())
+
       checker = syntax_check_policy_template_json.PolicyTemplateChecker()
-      if checker.Run(args, filepath) > 0:
+      if checker.Run(args, filepath,
+        original_file_contents, current_version) > 0:
         return [output_api.PresubmitError('Syntax error(s) in file:',
                                           [filepath])]
     finally: