Update build system guidance
Update build system guidance

Remove references to the legacy closure compiler files and add
more guidance with respect to output locations and timestamps in
GN templates.

R=1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org

Bug: none
Change-Id: I672d196c7f2ceebf6e057c60c4b1630ae2887d90
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3477111
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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