win: Use test::ErrorMessage() in test::ChildLauncher
win: Use test::ErrorMessage() in test::ChildLauncher

Context:
https://chromium-review.googlesource.com/c/chromium/src/+/735820#message-ea316c39d1418af51ceb165cdfbd0d1724798466

Change-Id: I9ad3731ea4a049fd90e18afb0c210b3928f9f674
Reviewed-on: https://chromium-review.googlesource.com/738533
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>
Commit-Queue: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/crashpad,atom/crashpad,chromium/crashpad,atom/crashpad,atom/crashpad,chromium/crashpad