Skip midl.py verification on Windows 7

midl.py invokes midl.exe to verify the results. midl.exe then invokes
cl.exe. With the new Windows toolchain (updated VS and 10.0.19041 SDK)
the VC++ preprocessor fails on some Windows 7 machines for some reason,
specifically on the win32-chrome continuous builder. The failure cannot
be reproduced on a test Windows 7 machine so investigation is
difficult. Since Windows 7 is deprecated it seems reasonable to skip the
validation step when running on Windows 7 and earlier, which is what
this change does.

Bug: 1095767,1096231
Change-Id: If28ae351f48544427b54f5fae4a6b9ec362a7aca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2255126
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#780526}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 75a06bacff5fabe9d211c01c7f21eb0b5e5eafc0
1 file changed