This is necessary to appease "gn check" if gtest_prod.h becomes a part
of the Chromium checkout, instead of a third-party repository brought
over by Chromium's DEPS. The file is already listed in v8's DEPS, but gn
does not use DEPS as an input.

BUG=chromium:630705

Review-Url: https://codereview.chromium.org/2807353002
Cr-Commit-Position: refs/heads/master@{#44604}