Add section on the legacy debug_devtools GN flag
Add section on the legacy debug_devtools GN flag

R=1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org
CC=alexrudenko@chromium.org

Change-Id: Ie7978f92a3d68a30a1a7ca56c0e5ac85fd9d23e8
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2207321
Auto-Submit: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Reviewed-by: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>
Commit-Queue: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>

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