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This improves the assert message when user32.lib cannot be found during
gn gen on Windows. This is an ongoing source of problems for external
contributors and this change may help with diagnostics.

Change-Id: I1649fb4d9cac4a735377df14149f554b434d63cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2136889Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756952}