GN: Visual Studio generator does not depend on is_debug variable

R=brettw@chromium.org, tmoniuszko@opera.com
BUG=

Review URL: https://codereview.chromium.org/1734083002

Cr-Original-Commit-Position: refs/heads/master@{#377699}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5e369d0ddbe53298b2d59bd9af3f81ccffa84e0c