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For some trunk builders, they set env var CHROMIUM_TEST_LAUNCHER_BOT_MODE.
For unit test, we need to unset the var to test some scenario.
https://source.chromium.org/chromium/chromium/src/+/master:ui/base/test/skia_gold_pixel_diff.cc;l=153;drc=07e4c76dd0f6b37146a1c35e4a96fae53aff7371

Bug: 1146399
Change-Id: Ifb71831060a4bd779ef48e83ca8d66741e5e57e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523555Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825590}