Since this is to be shared between repos, I'll add a canned check that will verify if any .vpython files are modified in the patch and validate them.

RE #1, I'd allow the user to specify specific paths to the canned check in addition to ".vpython", since users can have custom files or other mechanisms:

See: https://chromium.googlesource.com/infra/infra/+/master/doc/users/vpython.md#specification-probing

I'd do any file ending in ".vpython". I'd also allow the user to optionally specify additional custom paths.

CL: https://crrev.com/c/738169

Re #3: updated CL above with your suggestion.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/depot_tools/+/eba8347018794e4bc42b5171a31fa85fbe60e8ce

commit eba8347018794e4bc42b5171a31fa85fbe60e8ce
Author: Sergiy Byelozyorov <sergiyb@chromium.org>
Date: Fri Nov 03 06:48:57 2017

Add CheckVPythonSpec for verifying .vpython file syntax

This check will be used in Chromium, V8 and Catapult PRESUBMIT scripts.

R=maruel@chromium.org, tandrii@chromium.org

Bug:  777893 
Change-Id: I2ca1e774b89787c4d3b5f336315d145571858864
Reviewed-on: https://chromium-review.googlesource.com/738169
Commit-Queue: Sergiy Byelozyorov <sergiyb@google.com>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Daniel Jacques <dnj@chromium.org>

[modify] https://crrev.com/eba8347018794e4bc42b5171a31fa85fbe60e8ce/presubmit_canned_checks.py
[modify] https://crrev.com/eba8347018794e4bc42b5171a31fa85fbe60e8ce/tests/presubmit_unittest.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/22e04d3ee4d33dd74c4d6f259e9f6e9988df7ab6

commit 22e04d3ee4d33dd74c4d6f259e9f6e9988df7ab6
Author: Sergiy Byelozyorov <sergiyb@chromium.org>
Date: Fri Nov 03 10:23:14 2017

Bump depot_tools version in build recipe config

Doing it manually because autoroller is not working.

TBR=tandrii@chromium.org

Bug:  777893 ,  781169 
Change-Id: I51fb2631fbf6618cbb148bf45e6b9dfadda2f352
Reviewed-on: https://chromium-review.googlesource.com/753150
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>

[modify] https://crrev.com/22e04d3ee4d33dd74c4d6f259e9f6e9988df7ab6/scripts/slave/README.recipes.md
[modify] https://crrev.com/22e04d3ee4d33dd74c4d6f259e9f6e9988df7ab6/infra/config/recipes.cfg

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/0a7a87f60864458242177f6adfb61ce0c26197cb

commit 0a7a87f60864458242177f6adfb61ce0c26197cb
Author: Sergiy Byelozyorov <sergiyb@chromium.org>
Date: Fri Nov 03 10:39:56 2017

Add vpython verification presubmit

R=machenbach@chromium.org

Bug:  chromium:777893 
Change-Id: I2816934494309da458589b7d9b0c46fba9beaf38
Reviewed-on: https://chromium-review.googlesource.com/741221
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49102}
[modify] https://crrev.com/0a7a87f60864458242177f6adfb61ce0c26197cb/PRESUBMIT.py

The following revision refers to this bug:
  https://chromium.googlesource.com/catapult/+/c0373828f8acd061dc9ef36b19e5c621e6844971

commit c0373828f8acd061dc9ef36b19e5c621e6844971
Author: Sergiy Byelozyorov <sergiyb@chromium.org>
Date: Fri Nov 03 16:18:33 2017

Add VPython checks to Chromium presubmit

These checks will validate the syntax and check that all specified packages
exist. Checks will only run when .vpython files are modified.

R=dnj@chromium.org, sullivan@chromium.org

Bug:  chromium:777893 
Change-Id: I1a8f51b38e75d7377d8d4c746d8c8fe967f771bc
Reviewed-on: https://chromium-review.googlesource.com/753461
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>

[modify] https://crrev.com/c0373828f8acd061dc9ef36b19e5c621e6844971/PRESUBMIT.py

The following revision refers to this bug:
  https://chromium.googlesource.com/catapult/+/8e036faef710ffec58adb1046cd8825205d57f57

commit 8e036faef710ffec58adb1046cd8825205d57f57
Author: Ned Nguyen <nednguyen@google.com>
Date: Fri Nov 03 20:51:45 2017

Revert "Add VPython checks to Chromium presubmit"

This reverts commit c0373828f8acd061dc9ef36b19e5c621e6844971.

Reason for revert: suspect breaking numpy import which broke catapult roll.

Original change's description:
> Add VPython checks to Chromium presubmit
> 
> These checks will validate the syntax and check that all specified packages
> exist. Checks will only run when .vpython files are modified.
> 
> R=​dnj@chromium.org, sullivan@chromium.org
> 
> Bug:  chromium:777893 
> Change-Id: I1a8f51b38e75d7377d8d4c746d8c8fe967f771bc
> Reviewed-on: https://chromium-review.googlesource.com/753461
> Reviewed-by: Annie Sullivan <sullivan@chromium.org>
> Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>

TBR=sullivan@chromium.org,dnj@chromium.org,sergiyb@chromium.org

Change-Id: Id980ff43dce36ab94081082cd6254e8873a544b7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug:  chromium:777893 
Reviewed-on: https://chromium-review.googlesource.com/754141
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>

[modify] https://crrev.com/8e036faef710ffec58adb1046cd8825205d57f57/PRESUBMIT.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/366b6489f34bfa140c48b2f4727c87593301e050

commit 366b6489f34bfa140c48b2f4727c87593301e050
Author: Sergiy Byelozyorov <sergiyb@chromium.org>
Date: Mon Nov 06 18:20:43 2017

Add VPython checks to Chromium presubmit

These checks will validate the syntax and check that all specified packages
exist. Checks will only run when .vpython files are modified.

R=dnj@chromium.org, jochen@chromium.org

No-Try: true
Bug:  777893 
Change-Id: I57a3e2e178f2483f2b96506c11600100d932036d
Reviewed-on: https://chromium-review.googlesource.com/753342
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514182}
[modify] https://crrev.com/366b6489f34bfa140c48b2f4727c87593301e050/PRESUBMIT.py

The following revision refers to this bug:
  https://chromium.googlesource.com/catapult/+/48ce5259abe08c49336d5365894cab6b0da344fb

commit 48ce5259abe08c49336d5365894cab6b0da344fb
Author: Sergiy Byelozyorov <sergiyb@chromium.org>
Date: Wed Nov 08 15:17:00 2017

Add VPython checks to Catapult presubmit

These checks will validate the syntax and check that all specified packages
exist. Checks will only run when .vpython files are modified.
 
R=dnj@chromium.org, sullivan@chromium.org
 
Bug:  chromium:777893 
Change-Id: I34b33723b155e99ed9cb5bd066cc97a78c3caacc
Reviewed-on: https://chromium-review.googlesource.com/756879
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>

[modify] https://crrev.com/48ce5259abe08c49336d5365894cab6b0da344fb/PRESUBMIT.py