warningsAsErrors should be falsed on Windows 7 because we may
legitimately get a warning about loading the old d3dcompiler DLL on that
platform. The logic for this was reversed in the initial change. This
change flips the logic, changes the cutoff OS version, and always sets
the flag to something.

Bug: chromium:920704
Change-Id: Ibf8a7c9a48bffc183479b7614148d6c00c122a07
Reviewed-on: https://chromium-review.googlesource.com/c/1496015
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Jamie Madill <jmadill@google.com>