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gn has replaced gyp quite a while ago, so this shouldn't be needed any more,
and it'll make running gn a few milliseconds faster.

BUG=none

Review-Url: https://codereview.chromium.org/2735903005
Cr-Commit-Position: refs/heads/master@{#455277}

This change moves the list of files that are allowed to call
exec_script() into a variable in a file in the //build directory,
so that the list can be shared more easily by other repos that
are pulling in //build via DEPS but have their own custom dotfile.

R=brettw@chromium.org, machenbach@chromium.org
BUG=617873

Review-Url: https://codereview.chromium.org/2512043002
Cr-Commit-Position: refs/heads/master@{#433764}