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This is a long-overdue change that removes all of the code
from //tools/mb for invoking GYP instead of GN. This also
updates the docs to remove most of the references to GYP.
The docs can probably be updated even more to talk about
other things that have been added, but I'll leave that
for a separate CL.

R=jbudorick@chromium.org

Change-Id: Ic03c43189ec6fe0cc567d84480a27bc969a4477d
Reviewed-on: https://chromium-review.googlesource.com/825586Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523978}