Run gn check in tempdir, not under source tree
Run gn check in tempdir, not under source tree

gn supports putting out dirs at absolute paths now, so we can generate
the build files for the presubmit in a tempdir chosen by python instead
of somewhere underneath the source tree.

R=viettrungluu@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/752343002

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